
DEPARTMENT OF COMMERCE 


Circular 


of THE 


Bureau of Standards 

S. W. STRATTON, DIRECTOR 


No. 38 


THE TESTING OF RUBBER GOODS 


[Fourth Edition] 
SEPTEMBER 28, 1921 

(Superseding 3d Edition, July 19, 1915) 



PRICE, 20 CENTS 

Sold only by the Superintendent of Documents, Government Printing Office 

Washington, D. C. 


WASHINGTON 

GOVERNMENT PRINTING OFFICE 
1921 














DEPARTMENT OF COMMERCE 


Circular 

of THE 

Bureau of Standards 

S. W. STRATTON, DIRECTOR 


No. 38 


THE TESTING OF RUBBER GOODS 


[Fourth Edition] 
SEPTEMBER 28, 1921 

(Superseding 3d Edition, July 19, 1915) 



PRICE, 20 CENTS 

Sold only by the Superintendent of Documents, Government Printing Office 

Washington, D. C. 

WASHINGTON 

GOVERNMENT PRINTING OFFICE 
1921 
















% 


Q V 


< C> X 




LIBRARY OP CONGRESS 

5CSSVSD 

DEC 9-1921 

ooouiviLi\ r^i m,% i^iom 

. rriiMi.iMnihis.^ 







THE TESTING OF RUBBER GOODS 


* 



cT 




ABSTRACT 

This circular gives the methods used at this Bureau in the testing of rubber goods. 
The various physical tests commonly employed are described in detail and the ma¬ 
chines used for this purpose, many of which were designed at this Bureau, are 
illustrated and described in detail. Data are given showing the effect of various 
factors on the tensile properties of rubber. Special attention is given to the effect 
of temperature on the physical tests. The circular also contains a brief outline of the 
methods of collecting crude rubber and the processes used in the manufacture of various 
rubber articles. The methods used in the chemical analysis are given, together with 
an explanation of the reasons for making these tests and their significance. Regula¬ 
tions are given concerning the conditions under which tests are made for State and 
municipal governments. 


CONTENTS 


I. Introduction.. 

II. Materials used in the industry. 

1. Rubber—Sources, collection, and preparation. 

(a) Wild rubber. 

(1) Sources. 

(a) Euphorbiaceae. 

( b ) Apocynaceae. 

(c) Asclepiadaceae. 

(d) Urticaceae. 

( e ) Compositae. 

(2) Collection and coagulation. 

(b) Plantation rubber. 

(1) Sources. 

(2) Collection and preparation for market 

(c) Synthetic rubber. 

(d) Chemical and physical properties of rubber. . 

2. Gutta-percha and balata. 

3. Reclaimed rubber. 

4. Rubber substitutes.. 

5. Vulcanizing ingredients. 

6. Compounding ingredients. 

(a) Fillers. 

(b) Accelerators. 

(c) Pigments. 

III. Processes used in the industry. 

1. Washing of crude rubber. 

2. Drying. 

3. Compounding or mixing. 

4. Calendering. 

5. Frictioning. 

6. Vulcanization. 


Page 

6 

6 

6 

7 

7 

7 

7 

8 
8 
8 

9 

I r 

II 

11 

12 
12 

14 

*5 

16 

16 

16 

17 

18 

19 

20 
20 
20 
20 

22 

2 3 

24 


3 



































4 Circular of the Bureau of Standards 

Page 

IV. Manufacture of rubber goods. 25 

1. Solid rubber tires. 25 

2. Pneumatic tires. •... 27 

(a) Fabric. 27 

(b) Cord. 28 

3. Inner tubes. 29 

4. Plied hose. 29 

(a) Machine-made hose. 29 

(b) Handmade hose. 30 

5. Braided hose with rubber tube and cover. 31 

6. Cotton rubber-lined hose. 31 

7. Rubber tubing. 32 

8. Rubber belting. 32 

9. Molded rubber goods. 33 

10. Insulated wire. 33 

11. Boots and shoes. 34 

12. Rubberized fabrics. 34 

13. Druggists’ sundries. 35 

14. Rubber bands and thread. 36 

15. Rubber sponge. 36 

V. Testing of rubber. 36 

1. Physical testing of rubber. 37 

(a) Physical tests most commonly employed. 37 

(1) Comparison of normal and accelerated aging. 39 

(2) Tensile strength and ultimate elongation. 48 

(a) Separating rubber from fabric. 48 

( b) Emery wheel for grinding the surface of 

rubber. 49 

(c) Form and preparation of test pieces. 50 

(d) Measuring the thickness of rubber. 51 

(e) Grips for holding test pieces. 51 

(/) Testing machines. 53 

(3) Elasticity or “set”. 57 

(a) Machine for testing elasticity or “set”. . 57 

(4) Reduction in tension when rubber is held at a 

definite elongation. 58 

(a) Machine for testing reduction in tension .. 59 

(5) Conditions affecting the results of tension tests. 60 

(a) Influence of speed on tensile strength 

and ultimate elongation. 60 

( b ) Influence of temperature on strength, 

elongation, and “recovery”. 60 

(c) Influence of cross section on tensile 

strength and ultimate elongation.... 61 

( d ) Influence of the direction in which 

specimens are cut on strength, elonga¬ 
tion, and “recovery”. 62 

(e) Influence of “backing” on the tensile 

strength and “ recovery ” of hose 

lining. 63 

(/) Influence of previous stretching on 

strength, elongation, and “recovery”.*. 64 
(<?) Influence of the form ot test piece on 

the results of tension tests. 66 








































The Testing of Rubber Goods 5 

V. Testing of rubber—Continued Page 

1. Physical testing of rubber—-Continued 

(a) Physical testing most commonly employed—Continued 

(6) “ Friction” test. t . -1 

(7) Hydraulic-pressure test. 75 

(8) Steaming test. 76 

(9) Testing the rubber insulation of wire. 76 

(10) Comparative tests of machine and handmade 

tubes. 77 

(11) Testing of rubber bands. 79 

(а) Under one-fourth inch in width. 79 

(б) Bands one-fourth inch in width or over . 80 

( b ) Bureau of Standards procedure for physical testing of 

rubber. 81 

(1) Sampling. 81 

(2) Physical test methods. 81 

(a) Articles of irregular shape. 81 

( b ) Preparation of samples for test. 82 

( c ) Temperature of testing room. 83 

( d ) Preparation of tension test pieces. 83 

( e ) Tensile strength and ultimate elongation . 85 

(/) Set. 87 

(g) Friction. 88 

(h) Hardness. 91 

(i) Steam test. 92 

(j) Hydraulic-pressure test. 93 

( k) Interpretation of results. 94 

2. Bureau of Standards methods of chemical analysis. 95 

(a) Reasons for the analysis. 95 

(b) Sampling. 98 

(1) Preparation. 98 

(2) Classification. 99 

(c) Reagents. 99 

(d) Analysis. 100 

(1) Qualitative. 100 

(2) Quantitative. 100 

(a) General procedure. 100 

(b) Procedure for the analysis of 30 or 40 

per cent Para insulation. 107 

(c) Special determinations. 108 

( d) Joint Rubber Insulation Committee 

method. 118 

( e) Notes. 121 

VI. Testing of fabrics.. .. 122 

1. Normal atmosphere and moisture content. 122 

2. Weight. 122 

3. Threads per inch. 123 

4. Tensile strength. 123 

VII. Appendix. 123 

1. List of specifications. 123 

2. Bibliography. 124 

3. Table of specific gravities. 126 

4. Determination of the specific gravity, cost per pound, and cost 

per cubic foot of a rubber compound. 127 














































6 Circular of the Bureau of Standards 

I. INTRODUCTION 

The testing of rubber goods is a matter the importance of which 
is more generally appreciated now than formerly. The con¬ 
stantly increasing demand for rubber goods by the general public, 
automobile manufacturers, railroad companies, and other large 
consumers points to the necessity for developing standard specifi¬ 
cations and tests for rubber, as has been done in the case of iron, 
steel, cement, etc. 

The purpose of this circular is to describe the methods of testing 
used at this Bureau, with the hope that.sufficient interest may be 
aroused among manufacturers and purchasers of rubber goods to 
assist in bringing about that concerted action which is necessary 
for the standardization of tests and to furnish information which 
will enable the users of rubber goods to determine the quality of 
the materials that they secure. 

In order that one may readily understand the fundamental 
principles involved in these tests, this circular gives a brief 
account of the sources and preparation of the raw materials used 
in rubber manufacture and briefly describes some of the manufac¬ 
turing processes through which the materials pass. Without 
proper correlation of manufacturing processes, service rendered, 
and test results, the actual figures obtained by these tests are of 
little value. 

II. MATERIALS USED IN THE INDUSTRY 
1 . RUBBER—SOURCES, COLLECTION, AND PREPARATION 

Rubber is generally derived by a process of coagulation from 
a milky fluid (latex) contained in a special cell system (laticiferous 
system) of various trees, vines, and shrubs. The laticiferous 
system, which is distinct from the sap-bearing cell system, gen¬ 
erally lies between the outer bark and the cambium, and by cut¬ 
ting through the former the latex is obtained as a white to cream- 
colored, more or less viscous, fluid. This operation is termed 
“tapping.” The rubber is contained in the latex as small parti¬ 
cles which are generally suspended in the serum in the form of a 
negative emulsion. The rubber is separated from the latex 
either by evaporating off a part of the water or by a process of 
coagulation which varies according to the species, district, etc. 
After the coagulum has been separated from the serum, it is gen¬ 
erally purified and dried and is then exported. 


The Testing of Rubber Goods 


7 


(a) WILD RUBBER 


(i) Sources. —Rubber-bearing species are indigenous to con¬ 
siderable tracts of the tropical and subtropical zones of South 
and Central America, Asia, Africa, and Australia. The chief 
botanical orders are the Euphorbiaeeae, Apocvnaeeae, Urticaceae, 
and Compositae. 

(a) Euphorbiaeeae. —The most important genus of this order is 
the “Hevea,” and from the species H. brasiliensis about two- 
thirds of the world’s output is obtained. From this we get the 
famous Para rubber. There are two main districts in which fine 
Para rubber is prepared* (i) The “islands” at the mouth of the 
Amazon River, and (2) the “up river” regions, near and above 
Manaos. 

The scraps of rubber adhering to the trees and tapping cups 
are compressed into irregular masses and sold as “ negroheads. ” 
Up-river negroheads are generally termed “scrappy.” Island 
negroheads go by the name of “Sernamby. ” A third variety of 
negrohead, the Cameta, comes from the district of that name in 
southwest Para. 

Among the other varieties of H. brasiliensis may be mentioned: 
(a) Matto Grosso, fine and entrefine, 



( b ) Matto Grosso, virgin sheets, 


(c) Matto Grosso, negroheads. J 

(d) Mollendo (Bolivian Para). 

(e) Peruvian (fine, coarse, or scrappy). 

(/) Caucho (partly from Brazil, partly from Peru). Some caucho and Peruvian ball 
come from Castilloa elastica and Casiilloa ulei. 

Although the rubbers referred to above consist mainly of H. 
brasiliensis, the latices of other species of Hevea, such as spruceana, 
itaube, discolor, similis, and speciosa, are also employed to some 
extent in their preparation. 

Another genus of the Euphorbiaeeae is the Manihot. The most 
important commercial variety of rubber is the Manicoba or Ceara, 
which comes from the Province of Ceara, Brazil. 

(b) Apocynacece. —The bulk of African rubbers belong to this 
order, and the main genera are Funtumia, Landolphia, and 
Clitandra. 

The only species of Funtumia which is of commercial impor¬ 
tance is F. elastica. The natural habitat of the tree is on the 
Gold and Ivory Coasts in Uganda and in other parts of tropical 
Africa. The rubbers are known as Gold Coast lumps, Ivory 
Coast lumps, Niger niggers, Benin lump, Congo, and Cameroon. 


8 


Circular of the Bureau of Standards 


The species of Landolphia are all creepers or vines, which 
attain considerable size. They yield red • and black Kassai, 
Upper Congo balls, and Equateur from the Congo region; virgin 
sheets and pinky from Madagascar; Sierra Leone niggers, twists, 
and cakes, all from the Sierra Leone and southern rivers; and 
Conarkry, Soudan, and Bassam niggers and twists from French 
West Africa. 

The species of Clitandra are likewise vines and are largely 
distributed throughout Africa. 

In certain Brazilian Provinces the Hancornia, which yields 
the rubber known commercially as Mangabeira or Matto Grosso 
sheets, occurs in considerable quantity. 

(c) Asclepiadacece. —A rubber of high resin content termed 
Jelutong is obtained from species of Dyera, the most common 
being D. costulata. Jelutong varieties are among the commonest 
forest varieties in Borneo, the Malay Peninsula, and Sumatra. 
Pontianak is a variety of Jelutong grown in South Borneo. 

(d) Urticacece. —The most important species of this order occur 
in tropical Asia, Mexico, and South and Central America. 

Ficus elastica is found mainly in Asia (Burma, Ceylon, Malaya, 
Java, and India). The principal commercial brands are Assam, 
Rangoon, Java, and Penang. 

The species of Castilloa (principally C. elastica and C. ulei) 
represent the indigenous rubber trees par excellence of Mexico 
and Central America. We have Peruvian “caucho” rubber from 
Peru, Negro caucho from Ecuador, and Mexican strips and the 
different “West Indian’’ or Centrals from Costa Rica, Nicaragua, 
Honduras, San Salvador, and Guatemala. 

(e) Compositce. —The name guayule is derived from the Spanish 
“hay” and the Indian “hide” or rubber yielder. The rubber¬ 
bearing species is Parthenium argentatum. It is a gray woody 
shrub of spreading habit, growing generally between i and 3 feet 
high. It differs from other rubber-bearing plants in that it has 
no latex but contains the rubber in the cellular tissue of the 
epidermis and to a small extent in the branches and leaves. The 
plant, when cut at the roots, will send up new shoots if rain falls 
within a certain time. 

Guayule is now cultivated scientifically on plantations in Cal¬ 
ifornia and southern Arizona, and the next few years should show 
marked progress in the cultivation of the guayule shrub in the 
United States. 


The Testing of Rubber Goods 


9 


( 2 ) Collection and Coagulation. —In the Amazon district the 
trees are tapped by means of a small iron hatchet having a blade 
about 1 inch broad. The incisions are generally made in the form 
of V-euts or oblique lines. The first tappings are made at a height 
of from 6 to 7 feet, subsequent incisions at roughly to 2 inches 
below the first one until the base of the tree is reached. About 
35 consecutive daily tappings are therefore necessary to complete a 
tapping line. A fresh tapping line is then commenced at approxi¬ 
mately 18 inches from the first one. The latex is collected in 
small tin or earthenware cups fixed to the tree by means of moist 
clay. The daily yield of latex from a mature tree is about 1X 
ounces. On this basis the annual yield of rubber from the average 
Amazon tree is about 5 pounds. The latex is transferred to pails 
and is coagulated by a smoking process. 

In a small brazier a fire is made from palm nuts. A long 
wooden rod or paddle is so arranged, one end on a crosspiece, 
the other on the operator’s knees, that it can be rolled either over 
the top of the chimney and so be exposed to the full volume of 
smoke, or over the basin containing the latex. The operator 
pours a small quantity of latex over the wooden paddle and thus 
forms a thin film of liquid. This is rotated in the smoke until it 
sets. A fresh quantity of latex is then poured upon the first 
film, smoke is again applied, and so on until a “biscuit” or ball 
of rubber of the required size (20 to 100 pounds), consisting of 
innumerable thin layers tightly adhering to one another, is formed. 
The ball is then removed from the paddle and is ready for export 
as the “fine Para” of commerce. 

There is some difficulty in collecting the latex from the Manihot 
trees, owing to its viscous character and its property of rapidly 
coagulating. The natives allow the latex to coagulate naturally 
as it flows down the tree, and the bulk of the rubber is collected 
in tears and scraps which are stripped off. A certain amount of 
latex reaches the ground and this is collected on leaves or directly 
from the soil. 

The African rubber trees of the genus Funtumia are tapped in 
a manner similar to that used on the Hevea trees. The collecting 
is done entirely by the natives, and relatively little is known of 
their methods. Funtumia latex coagulates readily on boiling 
and most native methods are based on this fact. 

The methods employed by the natives for coagulating vine 
latices are of the most diverse kind. Thus, red Kassai is said to be 


io Circular of the Bureau of Standards 

obtained by smearing the latex on the body and allowing the 
natural heat to evaporate the water, after which the rubber is 
stripped off. Black Kassai is obtained by a combined boiling 
and smoking process. Some of the “ball” rubbers are obtained 
by applying a coagulant such as salt to the cuts made in the vine. 
The thread rubber thus obtained is wound into a ball of the 
desired size. More salt is also added at intervals so as to maintain 
constant coagulation. Vines do not readily lend themselves to 
tapping. They are generally cut down and bled to death. 

Jelutong is obtained from a number of species of Dyera. The 
trees grow to a very large size, those having a diameter of from 
4 to 6 feet being quite common. A mature tree will yield about 
ioo pounds of latex with 40 tappings per year. The latex is rich 
in solids and will yield about 65 per cent of wet but solid Jelutong. 
The latex is coagulated by the natives, who use curious mixtures 
of kerosene, copper sulphate, and alum. The solid matter in 
Jelutong, however, is quite largely resin and contains only a 
small percentage of rubber. 

The wild guayule shrub is generally collected by pulling up 
the entire plant, which is baled for shipment to the factory for 
extraction. Special methods are used to obtain the crude rubber 
because it is held in the cells of the plant. The dry plant yields 
about 9 per cent of pure rubber, although the percentage is often 
greater. Guayule that is cultivated scientifically gives a yield 
of from 10 to 20 per cent of the weight of the dry shrub. Three 
types of processes have been used for the extraction of rubber 
from the plant: (1) The alkali process, in which the shrub is 
boiled with a solution of caustic alkali; (2) the solution process, 
in which the rubber is extracted with carbon bisulphide or some 
other solvent; and (3) the mechanical process. The great bulk 
of guayule rubber is obtained by this latter process. The shrub 
is crushed and ground in pebble mills with water. The material 
is then run into settling tanks where the water-logged woody 
fiber sinks, while the rubber floats and is skimmed off. It is then 
sheeted and washed on rubber mills and dried. The rubber so 
obtained is quite dark in color on the surface and contains about 
20 per cent of resin. In quality it is about equal to the softer 
rubbers such as caucho ball and the softer crepes. When used 
with the better grades of rubber, as plantation sheets and crepes, 
excellent results are obtained. A large amount is also used in 
friction compounds. 


The Testing of Rubber Goods 


ii 


(b) PLANTATION RUBBER 

(1) Sources. —The plantation rubbers are obtained chiefly 
from Ceylon, the Federated Malay States, Dutch East Indies, 
Borneo, and the Pacific Islands. 

The tree which is now almost exclusively grown in these plan¬ 
tations is the Hevea brasiliensis. The Hevea grows in a narrow 
belt on both sides of the Equator, provided there is plenty of 
moisture. Before planting, a great amount of work must be 
done to clear the land, by cutting down the trees and under¬ 
brush, then burning it over when dry. The dead wood is removed 
and not allowed -to rot upon the ground. From this point on a 
vast amount of labor is required to keep out the weeds. The 
seeds are carefully selected from trees which give the greatest 
yield of rubber, and extreme care is taken in their propagation. 
One or two year old plants from the nurseries are set out from 
50 to 200 to the acre during the rainy season. The trees increase 
in height from 6 to 10 feet a year and in girth from 3 to 5 inches. 
They are not ready for tapping until 5 or 6 years old. The 
annual yield from such young trees is less than a pound of rubber 
each and gradually increases as the trees grow older. The mature 
trees yield up to 4 pounds of rubber per year. 

( 2 ) Collection and Preparation for Market. —The trees 
are tapped mostly on the herringbone system, which consists of 
a series of oblique cuts running into a central channel. In 
making the cuts strips of bark of one-thirtieth to one-twentieth 
inch are pared away. Tappings take place daily or on alternate 
days over certain periods. This is repeated until a certain area 
of bark has been removed. The portion of the tree so operated 
on is then allowed a period of rest sufficient for the renewal of the 
bark; for this three to four years appears to be an appropriate 
period. The latex is collected in a cup attached at the lower 
end of the channel. 

Plantation latex is generally coagulated by the addition of a 
small quantity of acetic acid. The coagulum is then passed 
through the washing rolls, which squeeze out much of the 
remaining mother liquor and wash out the excess of the other 
constituents of the latex. These rolls are of much the same con¬ 
struction as those used in the factory for washing. (See Fig. 1.) 
The treatment to which the rubber coagulum is subjected has an 
important bearing on the quality and marketability of the rubber. 
One of two courses is usually adopted: (a) The rubber is merely 


12 


Circular of the Bureau of Standards 


sheeted by the action of the washing rolls, by which process an 
appreciable quantity of the other constituents of the latex is 
retained. In order to avoid mold or tackiness, the rubber should 
be thoroughly smoked. (6) The rubber is converted into crepe 
and thoroughly washed during the process to remove the other 
constituents of the latex as far as possible. 

The rubber as it leaves the washing rolls is in sheets about one- 
eighth inch thick and io to 14 inches wide; the length may vary 
from a few feet up to 30 or 40 feet. These sheets are hung in a 
room at a temperature of 90 to ioo° F until they are dry. Some¬ 
times the room is kept full of smoke during the drying process. 
Almost any hardwood, coconut husks, or the like may be used to 
produce smoke. Smoke not only acts as an excellent antiseptic 
and preservative but seems to improve the strength of the rubber. 
The latter effect has yet to be explained. The sheets of rubber 
are pressed into blocks and are exported as plantation crepe or 
smoked sheets. 

It is interesting to note the tremendous development of the 
production of plantation rubber. In 1900 practically none was 
produced; in 1910 about 8000 tons and in 1919 over 285 000 tons 
were produced. The plantation acreage has increased from slightly 
over 1 000000 acres in 1910 to about 2 900000 in 1919. The 
annual production of wild rubbers from Brazil and Africa was 
about 60 000 tons during this period. Unless modern scientific 
methods are adopted this amount will not be increased. The 
United States used over 75 per cent of the world’s production of 
rubber in 1919. 

(c) SYNTHETIC RUBBER 

Synthetic rubber has been made by the gradual polymerization 
of the hydrocarbons butadiene, monomethyl butadiene (isoprene), 
and dimethyl butadiene (methyl isoprene). Each yields a differ¬ 
ent type of rubber of somewhat different composition and proper¬ 
ties. Such rubber oxidizes readily, requires accelerators for vul¬ 
canization, and its physical properties are comparable with those 
of the poorer grades of natural rubber. Unless synthetic rubbers 
can be made as good as the natural rubbers and as cheaply, they 
will not be manufactured commercially. 

(d; CHEMICAL AND PHYSICAL PROPERTIES OF RUBBER 

Rubber belongs to the class of compounds known to the chemist 
as hydrocarbons; that is, substances which contain only the ele¬ 
ments carbon and hydrogen. Harries, Weber, and other investi¬ 
gators determined that the formula for the rubber molecule is 


The Testing of Rubber Goods 


13 


polyprene, which is derived from the simpler hydrocarbon isoprene. 
The preparation of synthetic rubbers is based on the above work. 

Rubber can be made to combine with sulphur by a process 
known as vulcanization. In 1839 Charles Goodyear found that 
the physical properties of crude rubber are considerably altered 
by heating it with sulphur. At about the same time John Hancock 
found that by immersing rubber in a bath of molten sulphur 
vulcanization takes place. In 1846 Parkes discovered the cold 
cure process, which consists of immersing the rubber in a weak 
solution of sulphur chloride, or subjecting it to the vapors of such 
a solution. The solvent is generally carbon bisulphide or carbon 
tetrachloride. This method gives only superficial vulcanization, 
and can be used only on very thin sheets, for the rapidity of the 
reaction is so great that thick material will be overvulcanized on 
the surface by the time the interior is vulcanized. 

The beneficial changes that are brought about by the process 
of vulcanization are as follows: The strength, elasticity, and 
resilience of the rubber are increased, it loses its adhesiveness, is 
less affected by changes of temperature, and becomes insoluble 
in the ordinary rubber solvents. 

Attempts to explain the process of vulcanization have been 
made, but the various investigators do not agree. Thus Weber 
believed that vulcanized rubber is an “addition product” which 
is formed by chemical combination between sulphur and rubber 
hydrocarbon, whereas Ostwald believed that it is only a physical 
combination between the rubber and sulphur. It is not the pur¬ 
pose of this circular to attempt to discuss the different theories of 
vulcanization. 

The specific gravity of clean commercial rubber is about 0.92, 
but varies with the species, method of coagulation, and purity; 
the values obtained by various investigators are from 0.91 to 0.97. 
On warming, unvulcanized rubber becomes soft, then sticky, and 
finally melts. At temperatures below the freezing point of water 
it loses its elasticity and becomes rigid, and on immersion in 
liquid air becomes as brittle as glass. It is insoluble in water, 
but upon soaking in water for a long time it will absorb up to 
25 per cent. It dissolves in a number of organic solvents such as 
benzene, chloroform, carbon bisulphide, naphtha, etc. Rubber 
is a poor conductor of electricity and heat. It is not affected by 
alkalies, or weak hydrochloric and sulphuric acids, but concen¬ 
trated sulphuric acid and nitric acid of any concentration attack 
it. It gradually oxidizes in air with the formation of resins. 


14 


Circular of the Bureau of Standards 


2 . GUTTA-PERCHA AND BALATA 

These substances are not rubber but have many properties 
similar to those of rubber, and are used in the rubber industry. 

Gutta-percha is obtained from various trees, belonging to the 
natural order Sapotaceae, growing on the Malay Peninsula and the 
Archipelago. The trees of the species Dichopsis ( Palaquium) 
payena and Dichopsis polyantha are the main sources. 

The latex is collected generally by cutting down the tree and 
ringing the bark at intervals of 12 to 18 inches along the trunk. 
The milky fluid fills the grooves, soon coagulates, and is scraped 
off with a knife. Some latices do not coagulate quickly and these 
are collected in vessels. They are gently boiled with or without 
the addition of water. The raw gutta is cut up, softened in hot 
water, washed on a washing machine, and then forced through a 
strainer. After a second washing the inclosed water is forced out 
by a kneading machine and the mass is sheeted out in about 
5-foot slabs one-eighth to one-fourth inch in thickness. The 
above method of collecting the latex by cutting down the trees is 
extremely wasteful. 

Commercial gutta-percha is hard and tough, but on warming 
to 115 to 122 0 F it can be pressed into any shape, which it will 
retain on cooling. Its main use is for the insulation of submarine 
cables. A small amount is used for handles for surgical instru¬ 
ments and for golf balls. 

Gutta-percha consists of 30 to 84 per cent of “ gutta,” a rubber¬ 
like material, and 10 to 60 per cent of resin, which consists of albane, 
a crystalline resin, and fluavile, a yellow amorphous resin, in the 
ratio of about 2 to 1. 

Gutta-balata is obtained from the latex of the Mimusops balata, 
a tree growing in British, Dutch, and French Guiana. The 
commercial product comes in slabs about one-fourth inch thick, 
homy, and from a white to dark cream in color. It is collected 
in the same manner as gutta-percha, for which it is the only 
substitute. It has similar properties, although it is somewhat 
softer, owing to the fact that it contains more of the softer resin 
fluavile. It contains the resins in the proportion of approximately 
two parts of albane to three parts of fluavile. The main use 
of gutta-balata is in the manufacture of belting for power 
transmission. 


The Testing oj Rubber Goods 


15 


3 . RECLAIMED RUBBER 

The object in reclaiming rubber is to restore useful properties 
to the scraps of worn-out articles, so that the material may be 
used again. The aim is to restore to the vulcanized rubber the 
properties of crude rubber. This has not been accomplished, 
for no one has yet succeeded in removing the combined sulphur. 
All that can be done is to restore its plasticity so that it can be 
again worked and further vulcanized. The methods used are as 
follows: Alkali, acid, and solvent reclaiming. 

The alkali process was patented in 1899. The old rubber is 
ground and then treated with caustic-soda solution in a closed 
tank at a temperature of about 360° F for about 20 hours. The 
residue is then washed and dried. This treatment removes the 
fabric, free sulphur, and a portion of the fillers. 

The acid process was patented in 1881. The ground rubber is 
treated in a closed tank with sulphuric or hydrochloric acid of 
suitable strength in contact with live steam for from one to five 
hours. The residue is washed and dried. This treatment removes 
the fabric and dissolves all the acid-soluble fillers. 

The solvent process is misnamed. The oils added do not 
dissolve the rubber but merely soften and agglutinate it. In 
general, the finely ground rubber is heated with vegetable, resinous, 
or mineral oils to agglutinate the mass. The objection to this 
method is that it does not remove the free sulphur present and 
gives a very soft and sticky “reclaim.” This process is very 
little used to-day. 

Variations of the above processes have been tried; for instance, 
the fabric and free sulphur have been removed by alkali and then 
the resultant mass treated with an oil or other material like 
phenol or aniline. 

The properties of reclaimed rubber may be summed up as 
follows: 

1. No reclaimed rubber has yet been produced which is equal 
to good new rubber. 

2. Compounds made from reclaimed rubber are more or less 
inferior in strength and stretch to those made of new rubber. 

3. Good reclaimed rubber is a valuable ingredient in rubber 
goods of a moderate price and in which a large quantity of mineral 
fillers can not be employed. It also serves a useful purpose in 
producing better aging properties in compounds to be used 
under special conditions. 


16 Circular of the Bureau of Standards 

4 . RUBBER SUBSTITUTES 

No true rubber substitute—that is, no material possessing 
all the properties of rubber—has yet been produced. Synthetic 
rubber is identical in composition with crude rubber and so 
can not be called a substitute. The term “rubber substitute” is 
commonly applied to materials which are produced by vulcaniz¬ 
ing certain vegetable oils, such as rape, corn, and cottonseed 
oils, either by treatment with cold sulphur chloride or by heating 
with sulphur. The sulphur-chloride process produces a white 
substitute, while that made with sulphur is known as brown 
substitute. These substitutes are used in the manufacture of 

t 

cheap soft rubber . articles of low specific gravity. Their use 
tends to reduce the strength, elasticity, and wearing qualities of 
the finished product, and they are unsuited for use in articles 
which are subjected to high temperatures. 

The so-called mineral rubbers should not be considered forms 
or varieties of rubber. They are bituminous materials, either 
natural products, such as gilsonite, or other types of asphalt, or 
the crude tar residue from the distillation of petroleum. 

5 . VULCANIZING INGREDIENTS 

Since the time when Charles Goodyear patented the vulcaniza¬ 
tion of rubber by heating with sulphur, a large number of experi¬ 
ments have been carried out to vulcanize rubber by means of 
some ingredient other than sulphur. Vulcanization was attained 
by the use of chlorides, nitrates, nitrites, fluorides, bromides, 
iodides, sulphides, polysulphides, sulphites, and thiosulphates 
of nearly all of the common earths and metals, and also by the 
use of selenium, bromine, and iodine. None of these methods 
is in use. 

6 . COMPOUNDING INGREDIENTS 1 

The substances that are mixed with rubber in the manufac¬ 
ture of rubber goods comprise a list of materials of widely varying 
natures. The rubber compounder finds it advantageous to 
classify these substances into definite groups, although often 
there is some overlapping between them. 

What may be accomplished by the proper use of compounding 
ingredients is indicated by the fact that countless commodities 

1 The specific gravities of a number of the substances used in the rubber industry are given in the appen¬ 
dix. These results, which were taken from a number of sources, indicate how the specific gravities of these 
substances may be expected to vary. The methods for the determination of the specific gravity, cost per 
pound, and cost per cubic foot of a rubber compound are also given in the appendix. 





The Testing of Rubber Goods 


17 


of rubber are now made. The ingredients may be classified 
as follows: (a) Fillers (1, inorganic, 2, organic); (6) Accelerators 
(1, inorganic, 2, organic); (c) Pigments. 

(a) FILLERS 

Certain fillers when mixed with rubber add qualities to the rub¬ 
ber compound which make their use desirable. A number are 
added merely as cheapeners and impart no desirable properties. 
If an excess of fillers is used, their effect on the rubber compound 
may be detrimental. The cost per unit volume is often the 
determining factor in the use of one filler in preference to another. 

(1) Inorganic Fillers. —Among the inorganic fillers are those 
which when added to rubber impart certain definite properties, 
such as toughness, increase in tensile strength (within certain 
limits), hardness, compressive strength, increased insulating 
properties, and resistance to steam and abrasion. The following 
list contains the most frequently used fillers and some of their 
general properties: 

Zinc oxide, lithopone, carbon black, lampblack, and magnesium 
carbonate not only slightly increase the rate of vulcanization 
but have a decided toughening effect on the rubber. Zinc oxide 
also improves the insulating properties of rubber. 

Aluminum flake, whiting, barytes, barium carbonate, and 
china clay impart no special properties and are considered as 
being inert. 

Tripoli (infusorial earth), talc, and soapstone tend to make 
the compound dry and stiff. Talc and soapstone are used mainly 
for dusting sheets of unvulcanized rubber and molds to prevent 
sticking during vulcanization. Asbestine is used in heat-resist¬ 
ing compounds; powdered glass and pumice are used in erasers. 

( 2 ) Organic Fillers.— The organic fillers include a large 
number of oils, vulcanized oils, waxes, paraffins, bitumens, and 
pitches. They are used mainly to facilitate the mixing of the 
inorganic fillers with the rubber, to render the compound adhesive, 
to soften the texture of the vulcanized compound, to soften the 
compounds which have to go through a tubing machine, etc., to 
decrease the porosity, increase the resistance to water, gas, acids, 
and alkalies, to decrease the specific gravity, and in the manufac¬ 
ture of insulated wires and waterproof material. 

Blown asphalts, called mineral rubbers, asphalts, pine tar, and 
coal tar are used in the manufacture of insulated wires, and tend 



18 Circular of the Bureau of Standards 

to prevent the “flowering” out of the sulphur. They are also 
used, as well as resin and shellac, in compounds that are to be 
used as frictions. They may be called the adhesive softeners. 

Paraffin, ceresin, and ozocerite are used to make compounds 
work more smoothly during the mixing and if properly used 
improve the aging qualities of rubber compounds. Palm oil and 
vaseline also make compounds work more smoothly during the 
mixing. 

White, brown, and black substitutes impart to the finished 
product the soft velvety feeling of a purer rubber compound. 

Ground cotton, leather, cork, and wood pulp are used mainly 
in the manufacture of soles, in which lightness, nonslipping prop¬ 
erties, and increased porosity are desirable. 

Linseed oil, paraffin oil, and aluminum palmitate are used in 
the manufacture of waterproofing material. 

(b) ACCELERATORS 

Accelerators are substances which, when added to a rubber 
compound, decrease the time required for vulcanization. With¬ 
out their use the output of a factory would be much smaller and the 
cost of rubber goods proportionally higher. They may be classi¬ 
fied in two groups, inorganic and organic. 

(1) Inorganic Accelerators. —Litharge, magnesium oxide, 
and lime are the most frequently used and are especially good for 
the vulcanization of soft rubbers that are rich in resins, and to 
which have been added oils, waxes, and pitches. • White lead 
(basic lead carbonate), sublimed white lead (basic sulphate), and 
red lead are used to a less extent. 

(2) Organic Accelerators. —During the last few years the 
use of the organic accelerators has increased considerably because 
of the large number of investigations that have been carried out. 
Their importance lies in the fact that they are extremely active 
and decrease the time of vulcanization to a greater extent than 
the inorganic accelerators. However, because of their activity 
extreme care must be taken in their use to avoid overvulcaniza¬ 
tion. Many organic compounds may be used as accelerators. 
They may be classified as follows. 2 


2 Andrew H. King, Chemical and Metallurgical Engineering, 15, No. 5 . 




The Testing of Rubber Goods 


19 


Classification 


1. —Carbon bisulphide addition products with: 

(a) . —Aniline. 

(b) . —Dimethylaniline. 

(c) . —Tetrahydropyrrole. 

(d) . —Dimethylamine. 

2. —Ammonium compounds: 

(а) .—Ammonium borate. 

(б) .—Aldehyde ammonia. 

(c).—Quaternary ammonium bases. 

3. —Amino compounds: 

(a) .—Paraphenylenediamine. 

( b ) .—Tetramethylenediamine. 

(c) .—Sodium amide. 

(d) . —Naphthylenediamine. 

(e) .— 0 0 dimethyl trimethyleneamine. 

(/).—Trimethyleneamine. 

(g). —Benzylamine. 

4. —Piperidine and derivatives: 

(a) . —Piperidine. 

( b ) .—Methyl piperidine. 

5. —Quinoline and derivatives: 

(a) .—Quinoline. 

(b) .—Quinoline sulphate. 

(c) .—Quinosol. 

( d ) .—Oxyquinoline sulphide. 

6. —Miscellaneous: 

(a) .—Anthraquinone. 

( b) .—Urea derivatives. 

(c) .—Formanilides. 


However, the most commonly used are thiocarbanilide (diphenyl- 
thiourea), hexamethylenetetramine, paranitrosodimethylaniline, 
and aniline. 

(c) PIGMENTS 


These are essentially used for their pigmenting value, although 
some, such as zinc oxide, lithopone, carbon black, and lampblack, 
are used for the special properties that they impart to the rubber 
compound. Antimony sulphide is used as a sulphur carrier. The 
following are used merely as pigments: Red oxide, Indian red, 
Venetian red, ultramarine blue, cobalt blue, Prussian blue, indigo, 
chrome green, cadmium sulphide, vermilion, yellow ocher, and 
chrome yellow. 


20 Circular oj the Bureau of Standards 

III. PROCESSES USED IN THE INDUSTRY 

1 . WASHING OF CRUDE RUBBER 

The wild rubbers and some grades of plantation rubbers must 
be washed to remove mechanically mixed impurities, such as sand, 
wood, stones, fragments of plant tissue, salts, etc. The method of 
washing depends upon the type of rubber. Most of the planta¬ 
tion rubber, however, reaches the factory clean and ready for use. 

The rubber to be washed is placed in tanks of warm water until 
soft, after which the larger pieces are cut up by circular knives 
or powerful shears and torn up on the cracker. The cracker is a 
massive machine which has two corrugated hardened steel rolls 
(see Fig. i) which rotate toward each other at different circum¬ 
ferential speeds and thus produce a tearing action. The rubber 
comes out torn into ragged pieces. During the process a con¬ 
tinuous stream of water is. allowed to flow down on the rubber. 
If the rubber is very dirty, it is placed in a churn or beater washer. 
The churn washer consists of an oval tank in which a large paddle 
wheel keeps the rubber and water in continuous agitation, while 
the impurities settle to the bottom. After this the rubber is 
washed on a washer until it comes out in a continuous uniform 
sheet having the appearance of crepe. A washer (see Fig. i) is 
similar to a cracker, except that the corrugations are finer. 

2 . DRYING 

The method of drying depends upon the properties of the vari¬ 
ous rubbers. Those having sufficient strength are hung in sheets 
in a large dark room, through which a current of warm air is passed. 
The lower grades of rubber which are sticky and not strong enough 
to bear their own weight are dried on horizontal racks. By this 
method four weeks or more are required. It has been largely 
replaced by the more rapid method of vacuum drying bv which 
the rubber is dried in pans. Rubber is also dried under auto¬ 
matically controlled temperature and humidity. 

3 . COMPOUNDING OR MIXING 

The mixing of the rubber compound is carried out on a massive 
“mill,” similar to Fig. i, which consists of two smooth, polished 
cast-steel rolls which rotate toward each other, the back roll rotat¬ 
ing slightly faster. They are hollow and have steam and cold- 
water connections for the regulation of the temperature as required 
by different types of rubber compounds. The distance between 




The Testing of Rubber Goods 


21 



8 

O 


£ 

I S 


• | *5 

^ l 


O’ *T3 


&0 

g .3 

5 u 

o s 

TJ 

8 C 


-C> 

«C> 


d 

c 

B 

2 


M 


■2* 
”9-9 


"ts 


rd 3 

s° 

Sf a 


8 Cd 
o *“• w 
« »- 
- d o 


e o 

x 

? 1 


-£ s 

a £ 

C 3 £ 


H 


•2 5 

t 2 

-Cv X 

M <3 

Cij 5 


*0 

o 


O 

to 











22 Circular of the Bureau of Standards 

the rolls is adjustable by means of set screws in the front part of 
the frame. 

The rubber is placed on the rolls and adheres to the slower mov¬ 
ing front roll, which is the warmer. At first the rubber has the ap¬ 
pearance of a ragged sheet. Pieces which break off are caught in 
a pan set beneath the rolls and are again added to the mass. The 
mass gradually softens and becomes a smooth sheet, when it is 
ready for the addition of the fillers. The operation just described 
is called “breaking down” the rubber. The fillers are now gradu¬ 
ally added. The part which sifts through into the pan is again 
added to the mass. During the breaking down of the rubber and 
the mixing the operator slits the sheet with a short knife and folds 



it back on itself as it goes in between the rolls. The sulphur and 
accelerator are frequently and preferably added after the fillers 
have been incorporated with the rubber. The mixing is continued 
until a homogeneous mass has been obtained, when it is cut off 
in slabs, dusted with talc or soapstone, and sent to the storage 
room. 

4 . CALENDERING 

• 

Sheets of any thickness are produced by running the rubber on 
a calender (see Fig. i) which consists of three adjustable hollow, 
smooth, polished steel rolls, similar to those of a mixing machine, 
set vertically in a massive iron frame and rotated at the same rate 
of speed. The temperature required for various rubber compounds 
is secured by steam and cold-water connections to the rolls. The 










The Testing of Rubber Goods 


2 3 



rubber compound before going to the calender must be softened 
by working on a “warmer,” which is generally located in front of 
the calender and is similar to a mixing machine. The skeleton 
diagram in Fig. 2 shows the method of operation. 

The rubber from the warming mill is fed in between the two top 
rolls, which rotate toward each other. The rubber compound 
adheres to the middle roll until it is taken up by the running cloth 
from reel 1 which passes between the second and third rolls, which 
rotate toward each other. As the rubber comes in contact with 
the winding cloth, which is to keep it from sticking together, and 


Fig. 3 .—Vulcanizing boilers for the vulcanization of hose, tubing, etc., which are cured 

in open steam 

is carried to reel 2, a winding apparatus automatically winds the 
rubber and cloth on a wooden or metal drum. The desired thick¬ 
ness is obtained by building up the necessary number of layers. 
This process tends to avoid flaws in the finished sheet. 

5. FRICTIONING 

The “friction” is a layer of rubber which acts as an elastic 
bond to hold together layers of rubber and fabric or layers of 
fabric. The “ frictioning ” is carried out on the same type of 
machine as a calender; in fact, a calender is generally geared so 
that it can be used for calendering or frictioning. In calendering 












24 Circular of the Bureau of Standards 

the rubber is merely sheeted out to a definite thickness, whereas 
in frictioning it is forced into the meshes of the fabric. The forcing 
of the rubber compound into the fabric is accomplished by driving 
the bottom roll at a slower speed than the middle roll. 

6. VULCANIZATION 

Vulcanization is the term applied to any process that so combines 
a part of the sulphur with the rubber as to transform the raw 



Fig. 4.— Vulcanizing press equipped with constant temperature control 

This is used for the vulcanization of molded articles which are cured under pressure. The temperature 
and time of vulcanization are controlled automatically 

mixed rubber by physical and chemical reactions to the finished 
article. 

The different methods of vulcanization are: 

(а) Open Steam Cure .—The vulcanizer or heater (see Fig. 3), 
which consists of an insulated cylinder provided with steam and 
drip connections, is heated either by direct steam or a steam jacket. 
The rubber goods are placed in an iron carriage which is run into 
the heater on tracks, the door is closed, and the steam turned on. 
The temperature of the heater should be controlled automatically. 

(б) Press Cure .—The vulcanizing press (see Fig. 4) is used for 
molded goods. It consists of two or more hollow platens heated by 























I 


The Testing of Rubber Goods 25 

steam with automatic temperature control. They are forced and 
held together by hydraulic pressure, the rubber being contained 
in molds placed between the platens. 

(c) Cold Cure .—“Cold cure” consists in dipping the rubber 
article in a solution of 1 to 3 per cent of sulphur chloride in car¬ 
bon bisulphide or carbon tetrachloride. This method is used 
only for the manufacture of thin articles. 

(d) Bath Cure .—Vulcanization is also carried out by dipping 
the rubber articles into a bath of molten sulphur. 

( e ) Hot-Air Cure .—Vulcanization is carried out in large cham¬ 
bers heated by steam in which the air is thoroughly circulated. 

(/) Vapor Cure .—This is accomplished by exposure to the 
vapors of a solution of sulphur chloride. 

IV. MANUFACTURE OF RUBBER GOODS 
1. SOLID RUBBER TIRES 

Although there are several types of solid tires, the methods of 
manufacture are essentially the same in most respects. They 
all consist of a tread or wearing portion which constitutes the 
larger part, and a hard rubber base. In some tires there is a strip 
of rubber to act as a bond between the tread and the base. The 
base is squirted through a tubing machine and comes out in the 
proper shape to fit the steel tire rim. A tubing machine (see 
Fig. 5) contains a great screw revolving inside a cylinder. The 
rubber is fed into the mouth of the cylinder and is forced out 
through a die which gives it the proper shape. The opening 
through which the rubber comes out is heated by steam to make 
the rubber more plastic. The stock is delivered from a “warm¬ 
ing-up” mill directly to the tubing machine. After leaving the 
machine the tubed stock passes out on a table where it is cut 
into proper lengths. Th$ tread stock is tubed as described above 
or built up to the required thickness from calendered sheet which 
is wound over the hard rubber base. The steel rim is painted 
with rubber cement; then the base and tread are put on. After 
the base and tread have been placed upon the rim, the tire is 
allowed to rest at least a day before it is vulcanized. About 20 
tires are vulcanized at once, the molds being stacked one upon 
the other and closed by hydraulic pressure exerted by a plunger 
which passes up through the bottom of the steam-heated vul- 
canizer. The “cure” or time required to vulcanize tires is several 
hours, the exact time depending on the nature of the stocks. 


26 


Circular of the Bureau of Standards 


Fig. 5 



. Machine for making seamless rubber tubing by forcing the rubber compound 

through a die 












\ 


The Testing of Rubber Goods 27 

The cure is longer for solid than for pneumatic tires, because of 
the thickness of rubber through which the heat must penetrate. 

2. PNEUMATIC TIRES 

(a) FABRIC 

The fabrication of pneumatic tires consists in building up on 
a core the tire structure from the various parts which are dis¬ 
tinctive and are made independently. The building of the tire 
consists therefore in assembling these various essential parts, 
called the “bead,” the “frictioned fabric,” the cushion,” 
“breaker strip,” “side wall,” and the “tread.” 

The “bead” is the edge of the casing which holds the tire in 
place on the rim. There are two types of beads, the clincher 
and the straight side. The clincher bead is usually made of 
rubber which is run on a tubing machine and partially cured 
before going into the tire. The straight-side bead is rubber rein¬ 
forced with strands of steel wire. A coat of cement is applied, 
and layers of fabric are put on. The beads are now ready for 
tire building. 

The tread is run on a tubing machine or on a calender designed 
to give the desired shape. The side wall is cut from a calendered 
sheet. 

The fabric which is to be frictioned is first dried by passing it 
slowly over steam-heated rolls. While still warm it is delivered 
to the frictioning calender. When cloth is to be frictioned on 
both sides, it is sent back through the same process. As it comes 
from the calender a “liner” is wound up with it to prevent the 
rubber from sticking. Besides being frictioned on both sides, the 
fabric is “skimmed” on one side; that is, a thin layer of rubber 
is calendered on. The fabric is now taken to bias cutters, which 
cut definite widths on a 45 0 angle with the warp. The machine 
operates as follows: A row of automatic fingers grasps the edge 
of the fabric and pulls it forward the required distance. Then a 
knife drops and cuts off a strip. The fingers release the strip 
and return to grasp another width of fabric. The fabric is un¬ 
wound automatically from the roll as it came from the friction¬ 
ing calender and a slack is maintained between the unwinding 
device and the cutter. The pieces as they are cut off drop upon 
an endless belt which carries them to the operator, who picks 
them up and places them between layers of fabric. They are 
now taken to the “splicer,” who laps the ends and presses them 


t 


28 


Circular of the Bureau of Standards 

down with a roller. The continuous lengths are now wound upon 
rolls with a liner between the layers of fabric, which is now 
ready for tire building, the general procedure being as follows: 

Tires are built on iron cores having the shape and size of the 
inside of the tire. The tire-building machine is.arranged so that 
the core can be revolved and the frictioned fabric fed from rolls 
on adjacent racks. About half of the total number of plies which 
are to be used in making the tire are put on. In doing this, care 
is taken that no seams shall come over each other. Each ply is 
rolled down smooth before applying the next one. After this has 
been done the bead is put in place on each side and the remaining 
plies are put on. The plies of fabric are worked around the bead, 
and the side wall is put in place. A strip of practically pure 
rubber, called the “cushion,” goes on top of the fabric. On the 
cushion is placed a “breaker strip” of coarse loosely woven fabric, 
and finally the tread is applied. It is good practice to have the 
work inspected after each operation in order to detect any defects 
in workmanship which might otherwise be concealed in the fin¬ 
ished tire. Tires are vulcanized on the iron cores in molds which 
have depressions to produce the charactistic design of the tread. 
The vulcanizer is the same as that used for solid tires. 

(b) CORD 

Cord tires are manufactured substantially the same way as 
fabric tires. Cord fabric is composed of parallel warp cords laid 
close together without the usual filler strands other than small 
threads of soft, light yarn, spaced about one-half inch apart. 
These threads act as a temporary support for the warp cords to 
hold them in place until the tire has been fabricated. 

The cord fabric is run over steam-heated rolls to dry it com¬ 
pletely, and is then frictioned on a calender or run through a 
trough containing a solution of rubber friction stock. In the latter 
case the fabric is dried by being conducted over steam-heated 
coils or rolls. It then passes into a second tank of rubber solution, 
is dried thoroughly, and wound up with a liner.* The cord fabric 
is then cut on an approximately 45 ° bias and the plies ^re butted 
instead of lapped as in the fabric construction. The tires are 
cured over an inflated bag which is similar to an inner tube rein¬ 
forced with two or three layers of fabric. After the molds are on, 
the bags are inflated and the tires cured in the same manner as 
fabric tires. 


The Testing of Rubber Goods 


29 


3. INNER TUBES 

The method of manufacture of inner tubes varies. In general, 
the rubber sheet which has been calendered to the proper thickness 
is cut to the desired width and length. This is wrapped one or 
more times around a steel tube called a “pole” and the edge 
rolled down, so as to form a tight seam. The poles are piled in 
racks and the tubes cured in steam, the method being similar to 
that described under “plied hose.” After they are removed from 
the vulcanizer the tubes are drawn off the poles by inflating them 
with compressed air in such a manner as to turn them inside out. 
The ends, if not already tapered during the process of manufac¬ 
ture, are placed on mandrels and forced against a wet, high-speed, 
circular knife which skives them so that they can be smoothly 
spliced without much increase in thickness. The tapered ends 
are roughened on a wire buffing wheel. At the point where the 
valve is to be inserted the tube is reinforced with a valve patch, 
which consists of an oval-shaped piece of rubber, generally with 
two plies of fabric inserted. The valve hole is punched and the 
valve screwed on. The ends are now placed on special sleeves 
and coated with rubber cement. After the cement has dried 
they are brushed with a dilute solution of sulphur chloride, and by 
means of compressed air one end is slipped over the other end and 
strapped dow r n firmly with a rubber strip until properly vulcan¬ 
ized, after w r hich the strip is. removed. The ends are less fre¬ 
quently cured by heat. The tube is then inflated and immersed 
in water to detect any leaks. 

4. PLIED HOSE 

(a) MACHINE-MADE HOSE 

For hose of small diameter it is usual to form the tube by pass¬ 
ing the rubber compound through a tubing machine. The com¬ 
pound is first softened on a warming mill, generally situated near 
the tubing machine. The tube as it comes from the nozzle of the 
machine is carried away on an endless belt which is adjusted to 
run at the proper speed. The desired lengths of tube are cut and 
talc blown in, if this has not already been done as it emerges from 
the die. These tubes are placed on steel mandrels by a rather 
ingenious process, as follow's: 

The mandrel, which is about 52 feet long, is placed on an endless 
belt and held stationary. One end of the tube having been 
placed over the mandrel, sufficient air pressure to expand the tube 


30 


Circular oj ike Bureau of Standards 


slightly is applied at the other end. The belt is now set in motion, 
and the tube as it is fed upon the belt floats over the mandrel on a 
cushion of air. 

Canvas for use in making rubber hose is usually cut on the 
bias from strips 40 to 42 inches wide into pieces long enough so 
that when placed end to end and lapped the resulting strip is 
just wide enough to produce the necessary number of plies on the 
hose. 

The tube is wiped off with a rag moistened with gasoline and 
the frictioned fabric applied. The frictioned fabric is wrapped 
on by a machine which consists of three rolls of about 2 inches in 
diameter and slightly over 50 feet long. The two bottom rolls 
lie in the same horizontal plane, and the top roll, which is just 
above and between the other two, can be raised while the pole or 
mandrel carrying the tube to be wrapped is being placed on the 
bottom rolls. After the mandrel is in place the top roll is lowered 
and thus held firmly between the three rolls. A rotary motion 
imparted to the rolls causes the tube to revolve, and the fabric 
and the rubber cover, which is attached to the fabric, are wrapped 
on in a few seconds. Before going to the vulcanizer the hose is 
wrapped with wet cloth. First a long strip is wrapped length¬ 
wise on the hose and over this a narrow strip is wrapped spirally. 
This is done in a machine similar to that used for wrapping the 
frictioned fabric and rubber cover around the tube. The narrow 
strip is held under tension and guided by hand. The hose is now 
placed on racks set in a carriage and run into the vulcanizer and 
cured. The steam pressure and time necessary for vulcanization 
depend upon the composition of the rubber compound, size of the 
hose, and the use for which the hose is intended. 

After vulcanization the wrapping cloth is wetted, stripped off, 
and the hose is removed from the mandrel by means of compressed 
air. The couplings are now put on and the hose sent to the store¬ 
room. 

(b) HANDMADE HOSE 

For hose of a diameter greater than 1 J < inches the tube is usually 
made from a strip of sheet rubber calendered to the proper thick¬ 
ness. The sheet is skived or cut with a tapering cut and wrapped 
around a mandrel by hand so that the edges overlap. They are 
pressed flat by means of a small roller to make a perfect seam. 
The frictioned fabric is cut as above for tubed hose, laid over the 
rubber tube on the mandrel, and gradually rolled around it by 


The Testing of Rubber Goods 


3 i 


means of a small hand roller until the required number of plies 
have been made. The rubber cover is made from a sheet of cal¬ 
endered rubber of the proper width to pass around the hose once 
and form a narrow lap, and put on in the same manner as the 
fabric except that the lap is carefully rolled down to form a per¬ 
fect seam. In suction or high-pressure hose, wire is generally 
used to strengthen the walls. It is wound on spirally and strips 
of rubber used to fill the spaces between the turns of wire. The 
hose is generally wrapped with a long strip lengthwise on the hose 
and then with a narrow strip wound spirally by rotating the hose 
on roller bearings. The narrow strip is held under tension and 
guided by hand. The hose is then finished as described above 
under tubed hose. 

5. BRAIDED HOSE WITH RUBBER TUBE AND COVER 

Another type of hose is made by passing a rubber tube, which 
is distended by moderate air pressure, through a bath of cement 
and then to the braiding machine where the first ply of fabric 
is braided over fresh cement. The dipping and braiding are 
repeated until the desired number of plies have been formed. 
Then the rubber cover is put on and the hose is vulcanized in 
a mold. While being vulcanized the hose is subjected to air 
pressure from within, which forces the rubber well into the meshes 
of the loosely braided fabric. Braided hose is wrapped and vulcan¬ 
ized in open steam under air pressure. 

6. COTTON RUBBER-LINED HOSE 

In the manufacture of woven cotton hose with a rubber lining 
the tube is made by hand in the usual way, and partially vulcanized 
in order that it may develop sufficient strength to be drawn 
through the cover. In the better grades the semicured tube is 
covered with a thin layer of softer compound known as the 
“backing.” In the cheaper grades of hose the tube is sometimes 
merely coated with rubber cement. A long slender rod is passed 
through the cover and carries with it a stout cord which is 
attached to the end of the rubber tube. This tube is now drawn 
through the cover. The ends are clamped over cones and the 
hose is filled with steam under pressure. This expands the tube, 
forces the backing into the fabric, and vulcanizes the rubber. 
The vulcanization is carried out on an inclined table in order 
that the condensed steam may escape through a trap at the lower 
end. 


32 Circular of the Bureau of Standards 

7. RUBBER TUBING 

Rubber tubing, for which there is a large demand, is made 
either with a tubing machine, as described on page 29 (tubed hose) 
or from calendered sheet as described on page 29 (inner tubes). 
Compounded tubing which is most extensively used for general 
purposes is made by the former method. Dies and cores of 
different sizes are provided. They may be interchanged to pro¬ 
duce any diameter and thickness of wall within the machine’s 
capacity. 

Pure gum tubing is usually made from calendered sheet, but 
it is sometimes run on a tubing machine. Its production by the 
latter method is rather difficult and requires careful supervision. 

Tubing is vulcanized in open steam. The lower grades, through 
which talc has been blown to prevent adhesion of the walls, are 
coiled up and imbedded in pans of talc and vulcanized. The 
better grades of tubing, which become very soft during vulcani¬ 
zation, have to be placed on mandrels and wrapped with sheeting. 
They are vulcanized as described on page 30. 

8. RUBBER BELTING 

Duck for rubber belting is passed over steam-heated rolls to 
remove the moisture and is then frictioned on both sides, as 
described under manufacture of pneumatic tires. The frictioned 
duck is cut lengthwise into strips, the width of which depends 
not only on the size of the belt, but also on the method of manu¬ 
facture. The duck is cut to any desired width by an arrangement 
of adjustable circular knives. 

One method is to make the inner plies of the belt with strips 
which are equal in width to that of the belt. These strips, 
stacked one above the other, are placed in the center of a strip of 
double -the width and in this position they are drawn through an 
opening with flared edges which folds the bottom strip over the 
others and forms a butt joint on the top face of the belt. The 
belt then passes between rolls which press the plies firmly together 
and at the same time lay and press a narrow strip of rubber over 
the joint. When the belt is to have a rubber cover, this is gen¬ 
erally calendered on the outside ply of fabric before it is put on. 
Rubber-covered belting is necessary onlv for conveyors and a 
few other special uses. 

In another method the plies are made of folded strips. The 
first strip is folded upon itself, as described above, so that its 


The Testing of Rubber Goods 


33 


edges form a butt joint. This folded strip is placed with the 
joint down upon the next strip which is, in turn, folded to form 
a butt joint on the back of the first strip. In this way the belt 
is built up with the desired number of plies. Where there is an 
odd number of plies the first strip is not folded. The last joint is 
covered with a narrow strip of rubber which is rolled flush with 
the surface. 

The belt is vulcanized while it is stretched and held under heavy 
pressure between the steam-heated faces of a long hydraulic press. 
This drives the friction into the meshes of the duck, vulcanizes 
the rubber, and prevents the belt from stretching excessively 
when in use. 

9. MOLDED RUBBER GOODS 

A large variety of rubber goods, such as valves, heels, fiber 
soles, mats, erasers, tiling, etc., are vulcanized in molds under 
hydraulic pressure. They are usually made from calendered 
sheet built up to the required thickness. Pieces are cut from the 
sheet either by hand or machine by means of dies of the approxi¬ 
mate size of the finished article. The pieces are trimmed to a 
definite weight so that they all have the same volume before they 
are placed in molds and cured. 

10. INSULATED WIRE 

Vulcanized rubber is used in large amounts for electrical insula¬ 
tion. The wire, which is generally coated with tin, is covered 
with rubber in one of two ways: The rubber may be “squirted’’ 
on the wire through a machine resembling the tubing machine, or 
it may be calendered into thin sheets which are cut into strips and 
pressed around the wire by means of a coating machine. Small 
wires are wound in layers on slightly tapered drums and then 
vulcanized, but the larger sizes must first be wrapped with tape 
frictioned on one side so that the insulation may not be pressed 
out of shape. Vulcanization is carried out in open steam. It is 
of great importance that the steam throughout the vulcanizer be 
dry and circulate uniformly around the wire. After vulcaniza¬ 
tion the wire is sometimes protected from injury by a covering of 
braid. If waterproofing is desired, the braided wire is dipped 
in melted tar and paraffin. 

56597°—21-3 


t 



34 


Circular of the Bureau of Standards 


11. BOOTS AND SHOES 

Rubber boots and shoes are made up on lasts similar to those 
used for leather shoes. The sole stock is calendered to the proper 
thickness, during which operation the tread design is embossed 
upon it. The soles are stamped out by steel dies either by hand 
or in a stamping machine. The upper stock is made by calen¬ 
dering the fabric with a thin sheet of rubber. The calendered 
sheet is not rolled up like others because of its stickiness and the 
undesirability of having the impression of the cloth upon it. 
It is cut off in the required lengths as it comes from the calender 
and the pieces spread out on cloth-covered racks. The various 
parts of the shoes are cut from these according to pattern. 

The inside includes inner soles, half soles, heel pieces, and instep 
pieces, which are covered by the upper and the sole. The last is 
painted with rubber solution and the linings of the uppers are 
stretched over it, the inside portions are laid on, the inner sole put 
in place, and the seams carefully rolled down. The half sole is 
put on and then the instep pieces, heel pieces, counter, the upper, 
and finally the soles. 

The shoes are dipped in a varnish which generally consists of 
blown linseed oil, sulphur, turpentine, and gasoline. They are 
placed on racks and cured by dry heat in large chambers heated 
by steam coils. The time of cure is comparatively long. 

12. RUBBERIZED FABRICS 

t 

In the manufacture of rubberized fabrics, the fabric is run 
slowly over a series of steam-heated coils to thoroughly dry it, 
after which it is kept in a drying room at a temperature of about 
6o° C (140° F) until it is to be spread or calendered. When the 
fabric is calendered no solvent is required, and the loss due to the 
evaporation is entirely avoided. Calendered fabric is more uni¬ 
form and it can be made more quickly because no time is required 
for drying. Calendering, however, requires more skill than 
spreading. 

A spreading machine consists of a horizontal roll supported on 
an iron frame. A long knife is so supported above the roll that it 
can be raised or lowered to obtain the desired thickness of rubber 
on the fabric. The rubber is mixed into a dough with a solvent, 
generally gasoline. A thin coating of rubber is spread on the fab¬ 
ric by placing the rubber dough in front of the knife as the fabric 
passes between the knife and the roll. The cloth now passes over 


The Testing of Rubber Goods 


35 


steam-heated pipes where the solvent is evaporated. The above 
operation is then repeated until the desired thickness of rubber is 
obtained. 

Rubberized fabric can be vulcanized by either the cold or the 
heat cure. The cold cure is carried out by allowing the rubber to 
come in contact with a dilute solution of sulphur chloride in carbon 
bisulphide or carbon tetrachloride. After the cure some magne¬ 
sium carbonate is dusted on to neutralize any acid that is formed. 
If the rubber compound contains reactive fillers, such as litharge, 
lime, magnesia, or zinc oxide, the cold cure must not be used. 
When a velvety finish is desired, the rubber is dusted with starch 
before it is vulcanized. 

For the heat cure the fabric is dusted with starch or talc and is 
wound on a steel drum. A sheet of tin foil is sometimes placed 
between the layers of fabric in order to facilitate the distribution 
of heat and to insure uniform vulcanization. When the cure is to 
be carried out in a vulcanizer, the drum is inclosed in a waterproof 
bag to keep the fabric dry. When dry heat is used no covering is 
necessary. In some factories the fabric passes from the spreading 
machine through the heated chamber at such a rate that the rub¬ 
ber will be properly vulcanized. Calendering is carried out in the 
usual manner, and the rubberized fabric is cured as described 
above. 

13. DRUGGISTS’ SUNDRIES 

Druggists’ sundries comprise a great variety of rubber articles, 
a large part of which are dipped goods, which are manufactured as 
follows: The forms on racks are dipped into a solution of rubber 
in naphtha or benzene and dried, the operation being repeated 
until a film of the required thickness is obtained. . They are cured 
by immersion in a solution of sulphur chloride or by exposure to 
its vapors. They are then removed, usually turned inside out, and 
the inner side cured. 

Hot-water bottles, ice bags, water and air cushions are either 
molded or handmade. In the former case the article is vulcanized 
in a mold under hydraulic pressure. Handmade goods, in which 
rubberized fabric is sometimes used, are made up from parts cut 
according to pattern and vulcanized in open steam. 


36 


Circular of the Bureau of Standards 


14. RUBBER BANDS AND THREAD 

These goods are generally made from pure rubber, sulphur, and 
small amounts of fillers. Rubber bands are made either on a tub¬ 
ing machine or from calendered sheet. The calendered sheet is 
cut into widths, the edges of which are cemented together to form 
tubes. These are passed under a trip hammer which presses 
down the seam. The tubes are cured in open steam or under 
water. After vulcanization the tubes are cut, while wet, by re¬ 
volving cutters. The bands are dried in hot air, inspected, and 
boxed. 

The calendered sheet from which elastic thread is to be cut is 
wound with a liner upon a hollow iron drum and vulcanized under 
water. The rubber sheet is then unrolled, coated with thin shellac 
solution, and tightly rewound. The sheet is then cut into threads 
on a cutting lathe. The threads are then boiled with caustic-soda 
solution to remove the shellac and free sulphur, thoroughly washed, 
dried, and stored in the dark. 

15. RUBBER SPONGE 

Rubber from which sponge is made contains substances which 
volatilize during vulcanization. Ammonia and its salts are 
commonly used for this purpose. An interesting procedure is as 
follows: A mixture of amyl acetate, alcohol, and a little water 
is compounded with the rubber mixing. During vulcanization 
vapors are formed in bubbles throughout the mass. The rubber 
is then boiled in a dilute solution of caustic soda and at once run 
through rolls into boiling water. This process is repeated until 
the bubbles burst. The sponge is then washed and cut up. 

* V. TESTING OF RUBBER 

For a number of years this Bureau has been investigating 
rubber products for various branches of the Government service. 
The purposes were to determine the character of the materials 
used, how they met the conditions of service, how they could be 
improved in quality, and finally to develop specifications for the 
purchase of goods of the desired properties without unnecessarily 
restricting competition. This investigation could not be made 
without suitable laboratory tests for measuring the qualities 
desired. Old tests were adopted or modified, new ones were 
devised, and apparatus was designed. The investigation also 


The Testing of Rubber Goods 


37 


required knowledge of industrial processes, which was gained by 
visits to manufacturing plants. 

Physical tests measure the tensile strength, the permanent set 
after stretching, the strength of the adhesion of plies in built-up 
fabrics, and the various other physical properties which each 
specific material should possess. 

Chemical analysis can not of itself furnish information on these 
points until physical tests and actual service have demonstrated 
what compounds are best adapted to particular needs. In many 
cases chemical analysis furnishes the only available information 
as to the nature and uniformity of composition of rubber goods. 

Finished rubber products are not in a stable condition, but 
undergo change when exposed to heat, light, and air. It is there¬ 
fore desirable to learn, if possible, what is the probable life of a 
given product in actual service or in storage. Various accelerated 
aging tests have been devised, but the results obtained by them 
should be used with caution. When satisfactory aging tests are 
devised they should be incorporated in specifications. 

It was found that there were many different specifications for 
the same products. This state of affairs is unfortunate because 
it increases the cost without any material advantage. This 
Bureau is endeavoring to standardize specifications for rubber 
goods. 

In the following pages the methods and apparatus for making 
physical tests are described in detail. 

1. PHYSICAL TESTING OF RUBBER 

Rubber testing in the present stage of its development is not 
susceptible of very great refinement as regards measurement. 
The nature of the material is such that refinement seems of less 
importance than uniformity of methods, which is absolutely es¬ 
sential where the work of different laboratories is to be compared. 
A more general interest in this matter would result in a sub¬ 
stantial benefit not only to reputable manufacturers and large 
consumers, but also to the public. 

(a) PHYSICAL TESTS MOST COMMONLY EMPLOYED 

The different properties that have been found desirable in rub¬ 
ber intended for different purposes have naturally given rise to 
numerous tests, of which the most widely applicable are the 
various tension tests. These tests in various forms are used to 
determine the more important physical properties, such as ten- 


38 Circular of the Bureau of Standards 

sile strength, ultimate elongation, elasticity, and reduction in 
tension when held at a definite elongation. 

In the case of such materials as pneumatic tires, rubber hose, 
and rubber belting, which are built up with layers of duck 
cemented or frictioned together with rubber, it is customary to 
determine the friction or adhesion between the plies of duck as 
well as the quality of rubber. It is also usual to subject hose 
(particularly fire hose and air hose) to a hydraulic-pressure test, 
in order to detect any imperfections in materials or workmanship. 

An important test in the case of steam hose consists in passing 
steam at about 50 pounds pressure through a short length of the 
hose, in order to determine if the tube, cover, and friction are of 
suitable composition to withstand the effects of service condi¬ 
tions. This test usually lasts for about six days, the steam being 
turned off at night to allow the rubber to cool. A decided hard¬ 
ening or softening of the rubber, or a large decrease in the value 
of friction, as a result of steaming, is an indication of inferior 
quality. 

No absolutely reliable test (other than an actual service test) 
has been devised for rubber steam packing, but in many cases 
valuable information may be obtained by clamping a piece of the 
packing between metal plates and subjecting it to the action of 
steam at a pressure equal to or slightly above that under which 
it is to be used. A more satisfactory method is to clamp the 
packing in the form of a gasket between pipe flanges and apply 
the desired steam pressure from within. The test should last 
several days, the steam being turned off at night to see if the 
joint has a tendency to leak as a result of the cooling effect. This, 
however, practically constitutes a service test. 

The testing of tires, or rather the materials used in their con¬ 
struction, is done almost exclusively by manufacturers. Mani¬ 
festly it would be too expensive for the individual consumer or 
small dealer to sacrifice whole tires for the purpose of securing 
test pieces. The more progressive manufacturers, however, real¬ 
ize that money expended in thorough and careful testing is more 
than justified by the increase in efficiency of their product. 

The tests which have been outlined above aid considerably in 
determining the quality of rubber goods. The problem of finding 
properly accelerated aging tests merits further investigation. 


The Testing of Rubber Goods 39 

One which has been used extensively consists in the application 
of drv heat. 

(1) Comparison of Normal and Accelerated Aging. —The 
object of this work was to find an accelerated aging test which can 
be used to determine how a vulcanized rubber compound will age 
under normal conditions. 

Twelve compounds were used. They were press-cured at 288° ± 
i° F. Each compound w T as represented by 42 sheets 3 measuring 
6 by 8 by ^ inch. Fourteen of these sheets were undervulcan¬ 
ized, 14 overvulcanized, and 14 properly vulcanized according to 
the best judgment of the manufacturer. The composition of each 
compound and length of cure are given in Table 1. 

TABLE 1.—Description of Test Samples of 12 Rubber Compounds 


Degree of vulcanization 

Vulcanization period at 288° F, in minutes, for samples Nos. G 

-1 to G-12 

G-l 

G-2 

G-3 

G-4 

G-5 

G-6 

G-7 

G-8 

G-9 

G-10 

G-ll 

G-12 

Undercure. 

15 

30 

30 

70 

5 

5 

15 

20 

10 

15 

15 

15 


20 

40 

45 

90 

10 

15 

20 

30 

15 

20 

20 

20 

Overcure. 

25 

50 

60 

40 

15 

20 

25 

40 

20 

25 

25 

25 








Ingredient 

Composition in parts per thousand 


450 

250 

450 

250 

300 

100 

100 


200 


200 


Coarse Para. 

100 

100 

100 


150 



200 


200 









100 


200 


200 



Reclaimed . 






150 



350 

Zinc oxide. 

200 

200 

200 

200 

200 

200 

200 

200 

200 

200 

200 

100 

Barytes. 

215 

215 

175 

175 


200 

160 

200 

200 

200 

200 

300 


100 

100 


100 

100 

100 

100 


100 

100 

50 

Whiting 



375 

175 

150 

175 

285 

285 

235 





40 

40 





50 

20 



100 

100 












20 






Sulphur. 

35 

35 

35 

35 

25 

25 

20 

25 

15 

15 

15 

30 









100 

















The tensile strength and ultimate elongation were determined 
at different periods: (1) After the samples had been normally 
aged by storing in a well-ventilated dark cabinet, at a tempera¬ 
ture of approximately 75 0 F, and (2) after being subjected to the 
effect of dry heat at a temperature of 160° F. The results ob¬ 
tained are plotted in Figs. 6 to 17, inclusive. 

3 These compounds were obtained through the courtesy of Dr. W. C. Geer, connected with a commercial 
rubber company. 












































































4 o 


Circular of the Bureau of Standards 


% 



Tima Days exposed a+/60‘F 

Fig. 6 . —Tensile curves for compound G—i. This and the following figures contain curves 

for the various compounds G—i to G—12 





























































The Testing of Rubber Goods 


4 i 



Tima Days exposed of !60°F ' 

For key to symbols for points on curves see G ~l 


Fig. 7 .—Tensile curves for compound G-2 






































42 


Circular of the Bureau of Standards 



For hey to symbols for points on curves see G~! 

Fig. 8 . —Tensile curves for compound G—j 













































The Testing of Rubber Goods 


43 



Time 


Days exposed at I60"F 

For key to symbols for points on curves see G "/ 


Fig. 9. —Tensile curves for compound G—4 
















































4 


Circular of the Bureau of Standards 



Time Days exposed atl60‘f r 

For key to symbols for points on curves see S~! 


Fig. io. —Tensile curves for compound G—$ 



For key to symbols for points on curves see S-t 

Fig. ii. —Tensile curves for compound G -6 






















































































The Testing of Rubber Goods 


45 



Time. Days exposed aH60*F 

For key to symbols for points on curves see G~! 

Fig. 12. —Tensile curves for compound G-y 



Ti me Days exposed at !60°F /6 

For key to symbols for points on curves see G“/ 

Fig. 13. —Tensile curves for compound G-8 
















































































46 


Circular of the Bureau of Standards 



Time Oays exposed at !60°F 

For key to symbols for points on curves see S~l 

Fig. 14 .—Tensile curves for compound G-g 



/ 2 3 4 S 6 7 S 9 II JZ 13 14 

e Days exposed at I 6 Q°F 

For key to symbols for points on turves see S-l 

Fig. 15 .—Tensile curves for compound G—10 













































































The Testing of Rubber Goods 


47 



"Time Days exposed of I60‘F 

For hey to symbols for points on curves see G'l 

Fig. 16. —Tensile curves for compound G-ll 



/ 2 3 4 S 6 7 8 3 /O // /2 /3 /4 

Time Days exposed at I6CTF 

For hey to symbols for points on curves see G'/ 


Fig. 17. — Tensile curves for compound G-12 


\ 

























































































4 8 


Circular of the Bureau of Standards 



From a study of these curves it may be concluded that— 

1. An apparent relationship exists between samples that have 
been aged normally and those that have been subjected to the 
action of dry heat at a temperature of i6o° F. Our experiments 
seem to show that subjecting a sample to dry heat at i6o° F for 
one day is equivalent to six months’ normal aging. 

2 . In all cases the relatively undervulcanized samples showed 
less deterioration and the relatively overvulcanized samples more 
deterioration than those which were properly vulcanized. 

A more complete series of rubber compounds should be sub¬ 
jected to the same tests in order to confirm these results. 


Fig. 18 .—Emery wheel for grinding the surface of rubber test pieces to produce a smooth 

surface 

( 2 ) Tensile Strength and Ultimate Elongation — (a) Sep¬ 
arating Rubber from Fabric .—When the material is made up with 
layers of fabric, as in the case of rubber hose, the first step in 
preparing specimens for the tension test is to separate the rubber 
from the fabric. Unless the frictioning is very poor, this will 
necessitate the use of a solvent. If there is more than one layer 
of fabric, the easiest way is to remove the first laver alone with 
the rubber. The rubber is then separated from the adjoining 
layer of fabric with the aid of gasoline blown from a wash bottle. 
Narrow strips are more easily handled than larger pieces and there 
is less danger of injuring the rubber. Great pains should be taken 
during this operation, because any flaw or local imperfection will 




The Testing of Rubber Goods 


49 



seriously vitiate the results. The rubber should be allowed to 
rest for several hours, in order that it may recover from the 
stretching it has received and that the gasoline may thoroughly 
evaporate. 

(6) Emery Wheel for Grinding the Surface of Rubber .—The strips 
thus removed retain the impressions of the fabric from which 
they have been separated, and it is necessary to grind the sur¬ 
face smooth in order 
to measure accurately 
the thickness of the 
test pieces. The emery 
wheel shown in Fig. 18 
was designed at this 
Bureau to accomplish 
this purpose, and has 
proven a very conven¬ 
ient and satisfactory 
arrangement. The 
wheel is operated by a 
one-eighth horsepower 
motor. The rubber to 
be buffed is clamped, 
as shown, to a carriage 
which is moved back 
and forth under the 5 
by iFf inch emery 
wheel (No.40),running 
at about 3000 revolu¬ 
tions per minute. The 
central portion of the 
carriage just under the 
rubber is slightly | 
raised, by which means 
it has been found that Fig 
the operation is more 
easily performed and with less danger of injury to the rubber. 
An adjusting thumbscrew serves to lower the wheel by very small 
amounts as the buffing proceeds. Shields are provided to keep 
the fine particles of rubber off the guide. The starting box, though 
not necessary, is desirable in bringing the wheel gradually up to 
full speed. The face of the wheel should be dressed at intervals 
as may be required to maintain a good abrasive surface. 


19 .—Arbor press and die for cutting rubber test 
pieces for determining the tensile properties 


56597°—21 


4 









50 


Circular of the Bureau of Standards 


(c) Form and Preparation of Test Pieces .—Test pieces are cut 
with a metal die, which not only saves much time, but also insures 
uniform width, which it is impossible to obtain if the specimens 
are cut by hand. An arbor press, Fig. 19, is perhaps the most 
convenient and satisfactory means of forcing" the die through the 
rubber, although many prefer to cut the test pieces by striking 
the die with a mallet. The operation is facilitated by wetting the 


A 






Fig. 20.— One-fourth and one-half inch test pieces with enlarged ends 

cutting edges of the die, and the rubber should rest on a smooth 
and slightly yielding surface which will not injure the cutting 
edges. A piece of leather is suitable for the purpose. The central 
portion of the test piece is straight, and the ends are gradually 
enlarged to prevent tearing in the grips of the testing machine. 
I'he w idth of the contracted section is usually made either one- 
fourth or one-half inch. Fig. 20 gives the dimensions of test 
pieces which have been found to give satisfactory results. Fig. 
21 show r s the various dies used at this Bureau. The one-fourth 


































































The Testing of Rubber Goods 


5i 


inch test piece is to be preferred for general use, for the reason 
that there is a less tendency for the rubber to tear in the grips or 
across the shoulder where the cross section changes. 

Parallel lines are placed on the specimens, and by means of these 
gage marks elongation and permanent extension are measured. 
A stamp consisting of parallel steel blades enables one to mark 
very fine lines with ink, without cutting the rubber, and in this 
way much time is saved and the chance of error very much 
reduced. 

In testing pure gum or compounds containing a large percentage 
of rubber, it is not practicable with a machine of convenient length 
to break test pieces having a gage length of 2 inches. In such 
cases shorter test pieces one-fourth inch wide over a gage length 
of 1 inch and 1 % inches wide at the ends are used. Test pieces 
in the form of a ring will be referred to later on page 66. 



Fig. 21 .—Shapes of dies used for cutting test pieces 


(d) Measuring the Thickness of Rubber. —A special form of 
micrometer caliper (Fig. 22, A) provided with enlarged contact 
surfaces and a ratchet stop is used to some extent for measuring 
the thickness of rubber and textile materials. The ratchet stop 
makes it possible to measure each specimen under the same 
pressure and thus adds to the uniformity of results. A spring 
micrometer (Fig. 22, B) is more generally employed for this 
purpose. It is exceedingly convenient and very easily read. 

(<?) Grips for Holding Test Pieces. —In testing rubber one of the 
greatest difficulties has been to grip the test piece in such a way 
as to prevent slipping, without at the same time injuring the 
rubber. Even a very small scratch on the surface of a rubber 
test piece is often sufficient to cause failure at that point. 

In order to prevent slipping of the test piece as its section is 
gradually ’--educed under increasing tension, it has been found 







52 Circular of the Bureau of Standards 

advisable to provide means for automatically tightening the grip. 
This may be conveniently accomplished by using a number of 
thin cylindrical disks, with knurled faces, mounted eccentrically 


Fig. 22. —Instruments for measuring the thickness of rubber. A, micrometer; B, spring 

micrometer 

on a pin (Fig. 23, A), which act independently, thus producing a 
uniform pressure over the gripping surface and preventing any 
uneven slipping. 


a b c 

Fig. 23. —Grips used, in making tensile tests of rubber. A, Eccentric disks which tighten 
automatically as the tension increases; B, spools with ball bearings for testing ring- 
shaped specimens 

When it is desired to test specimens of circular form, as in the 
case of rubber bands, jar rings, or washers, grips of the design 
shown in Fig. 23, B, may be used. Each grip consists of two rollers 










The Testing oj Rubber Goods 


53 



mounted on ball bearings. The 
action of these rollers is such that 
as tension is applied they rotate 
in opposite directions, thus tend¬ 
ing to equalize the stress around 
the circumference of the test 
piece. 

(/) T esting M achines .—The 
machine shown in Figs. 11, 12, 
and 13 is used for determining 
the tensile strength and ultimate 
elongation. The dynamometer 
1 (Fig. 24), having a capacity of 
125 pounds and graduated to 
one-fourth pound, is attached to 
the upper end of column 2, which 
is slotted to receive the rack 3, 
carrying the eccentric grip 4 at 
its upper end. The machine is 
operated by a one-twelfth horse¬ 
power motor, which is belted to 
the stepped pulley 5. A worm on 
the pulley shaft drives a worm 
wheel which is geared to a spur 
(not shown) inside of column 2. 
The eccentric grip 4 (Fig. 25) is 
attached to the dynamometer 
through pin connections 7 and 8. 
The plate 9 serves to prevent re¬ 
coil of the springs when a speci¬ 
men is broken, and acts in the 
following manner: The rod 10, 
which is rigidly attached to 
column 2, passes with very little 
clearance through a hole in the 
back of plate 9. The front of 
plate 9 is slotted to receive the 
rod 11, and is supported by the 
shoulder 12. As tension is ap¬ 
plied to the specimen, plate 9 is 
free to follow the shoulder 12, 
and passes down over the rod 10, 
but when the specimen breaks 


?IG. 24. — Motor-driven machine for test¬ 
ing the tensile strength of rubber. See 
Figs. 25 and 26for details 


















54 


Circular of the Bureau of Standards 



the upward pressure of the shoulder 12 causes the plate to bind 
on the rod 10, thus holding the springs under the maximum 
tension. The dynamometer is so constructed that the pointer 

remains at the maximum load when 
a specimen breaks. The load having 
been recorded, the upper grip is 
pulled downward (by means of the 
hand wheel 13 and hooks 14, if nec¬ 
essary, Fig. 24) to relieve the pres¬ 
sure of the shoulder 12 against plate 
9, and at the same time the rear end 
of the plate is raised to release the 
rod 10. The tension of the springs 
may now be relieved by allowing the 
grip to rise, and the pointer is re¬ 
turned to zero. 


Fig. 25. —Detail of iipper portion 
of machine shown in Fig. 24, illus¬ 
trating method of attaching grip 
and device for preventing recoil of 
spring 

At the end of a test the 
worm and gear are disengaged 
by means of the spring clutch 
15 (Fig. 26), and the rack is 
rapidly raised by hand to its 
initial position. 

The stepped pulleys pro¬ 
vide for different speeds to 
meet the requirements of ex¬ 
perimental work. 

Elongation between gage 
marks on the specimen may 


Fig. 26. —Detail of lower portion of ma¬ 
chine shown in Fig. 24, illustrating 
stepped pulley, gear drive, and releasing 
mechanism 


be measured on the scale 16, attached to column 2, but to do this 
requires the services of two operators. A simpler and equally 
accurate method is as follows: A wooden scale graduated to one- 















The Testing of Rubber Goods 


55 




Fig. 27 .—Illustrating method of 
measuring ultimate elongation 


Fig. 28.— “jSchopper” machine for test¬ 
ing the tensile strength of rubber 

This machine operates on the principle of a 
weighted lever 






















56 


Circular of the Bureau of Standards 



Fig. 29.— Motor-driven machine for test¬ 
ing rubber bands 

This machine operates on the same principle as 
the one illustrated in Fig. 24 


tenth inch, as shown in Fig. 27, 
is held in a slightly inclined 
position at the back of the test 
piece with its lower end pressed 
lightly against the rubber just 
back of the lower gage mark. 
As the rubber is stretched the 
eye follows the scale just back 
of the upper gage mark. With 
a little practice the elongation 
at break may be measured with 
a fair degree of accuracy. 

The Schopper machine illus¬ 
trated in Fig. 28 is worked by 
hydraulic power, its operation, 
briefly stated, being as follows: 
The rubber test ring is placed 
over the spools, and the lower 
spool is geared to the rack in 
such a way that it is caused to 
revolve during a test. This 
motion is transmitted to the top 
spool by the rubber test ring, 
the object of rotating the spools 
being to equalize the tension at 
all parts of the specimen. As 
the tension is increased, the 
weighted lever, to the short arm 
of which the top spool is at¬ 
tached, is gradually deflected. 
When the test ring is broken, 
the lever is held at the point of 
maximum load by means of a 
set of pawls, the breaking load 
being read from the curved scale 
and the elongation being indi¬ 
cated by the vertical scale just 
opposite the test ring. 

Fig. 29 shows a motor-driven 
machine of 15 pounds capacity 
which was designed for testing 
rubber bands. The load is ap- 





















The Testing of Rubber Goods 


57 


plied through the steel tape i, at the end of which is the grip 2, 
carrying spools 4, similar to those shown in Fig. 23, B. The 
graduated steel tape 3, attached to grip 2, with its zero point 
coinciding with the center of the spools 4, passes up behind the 
test specimen and through the column 5 to a reel just behind the 
spring balance. When the specimen breaks its elongation is de¬ 
termined by the distance between the centers of the spools, as 
shown by the tape 3. The plate 6 holds the springs under the 
maximum tension in a way already explained in connection with 
Fig. 25. Stepped pulleys provide for different speeds. When a 
specimen has been broken, the worm and gear are disengaged by 
means of the lever 7, and the bottom spools are raised by hand to 
their initial position. This machine is also provided with eccen¬ 
tric grips which are used for testing straight specimens of low 
tensile strength. 

(3) Elasticity or “Set.” —“Set” or “recovery” as applied 
to rubber is, in a way, synonymous with elasticity, and is measured 
by the extent to which the material fails to return to its original 
length after having been stretched. The set, together with the 
ultimate elongation and tensile strength, determines whether the 
compound has been properly vulcanized. For a given compound 
high set indicates undervulcanization, whereas a low set indicates 
overvulcanization. 

For example, if a test piece is stretched from 2 to 10 inches for 
10 minutes and then released, and if its length measured after 10 
minutes’ rest is 2.4 inches, the “set” under these conditions is 
0.4 inch, or 20 per cent. 

(a) Machine for Testing Elasticity or “Set .”—For merely 
stretching rubber to determine its elasticity or recovery after a 
definite elongation, without reference to the tension applied, the 
apparatus shown in Fig. 30 is used, in which six specimens may 
be tested at once. The spools 1 are free to slide on the shaft 2, 
and are slotted to engage pins 3 (not shown), which act as clutches. 
The movable grips are attached to three-fourth inch strips of 
leather-belt lacing which pass through clamps 4 and then to the 
spools 1. The action of these clamps is similar to that of an ordi¬ 
nary letterpress, and, with a one-half by three-fourth inch bearing 
plate, a moderate twist of the knurled head is sufficient to pre¬ 
vent any slip of the belt lacing when under tension. The operation 
of the apparatus is as follows: 

Six specimens being in the grips, one of the spools is moved 
along the shaft until it engages the corresponding pin, and the 



58 Circular of the Bureau of Standards 

shaft is revolved until the desired elongation measured between 
gage marks on the specimen is secured. The clamp is tightened 
to hold the specimen in this position, and the spool is shifted 
back so as to disengage the pin. The operation is repeated with 
each of the specimens in turn. Each specimen is released after a 
specified length of time (usually i or io minutes) and after an 
equal interval of rest the permanent extension or set is measured 
with a scale graduated to o.oi inch. 


Fig. 30 .—Apparatus for stretching rubber to determine its elasticity 

(4) Reduction in Tension When Rubber Is Heed at a 
Definite Elongation. —When rubber is stretched in a testing 
machine the extension gradually increases with the applied ten¬ 
sion, as is the case with other materials, but if the machine is 
stopped at any point so that the test piece is held fixed under a 
constant elongation, the tension in the test piece will gradually 
decrease. 






















Fig. 31 .—Apparatus for determininq the reduction in tension when rubber is stretched 

and held at a definite elongation 

capacity of 50 pounds, with 8-inch dials graduated to 0.2 pound. 
The lower grips are counterbalanced by weights, which are sus¬ 
pended from cords passing over pulleys just back of the spring 
balances. In this way each grip is held in an accessible position 
and is prevented from falling should the specimen break. 


The Testing of Rubber Goods 59 

The determination of this decrease in tension under specified 
conditions constitutes a test which is thought to be indicative of 
the quality of rubber and the extent of vulcanization. 

(a) Machine for Testing Reduction in Tension .—The apparatus 
shown in Fig. 31, which has a capacity of four test pieces, is used 
in conducting this test.- The spring balances are provided with 
live and dead pointers, which show the maximum tension as well 
as the tension at any time during the test. The balances have a 













6 o 


Circular of the Bureau of Standards 

( 5 ) Conditions Affecting the Resuets of Tension Tests.— 
In the absence of uniform methods of testing, it is found that 
results obtained in different laboratories sometimes show marked 
discrepancies which are due to the varying conditions under which 
the tests are made. 

(a) Influence of Speed on Tensile Strength and Ultimate Elonga¬ 
tion. —The speed at which rubber is stretched probably affects 
the results to a less extent than is often supposed, though doubtless 
different rubbers are not equally affected. 

Table 2 shows the results obtained in a comparative test of 
four compounds. 

TABLE 2.—Tensile Strength and Ultimate Elongation as Affected by Rate of Stretching. 


(Gage length=2 inches] 


Specimen 

Rate of 
stretch 

Tensile 

strength 

U ltimate 
elongation 



Lb./in. 2 

Per cent 


f 5 

2495 

605 


] 25 

2690 

635 


1 45 

2720 

635 


f 5 

1900 

455 

M 1. . 

| 25 

1940 

500 


1 <5 

1970 

490 


f 5 

375 

310 

B-l . . 


430 

350 


1 45 

465 

375 


f 5 

340 

105 

Gy-1 . 

| 25 

390 

115 


45 

430 

120 


These results would indicate a general tendency toward higher 
values for both tensile strength and ultimate elongation as the 
speed of stretching increases. 

(6) Influence of Temperature on Strength, Elongation, and 
“ Recovery .”—It is generally recognized that the physical proper¬ 
ties of rubber are affected by changes in temperature, though, of 
course, to a less extent after vulcanization than before. 

Tig. 32 shows the results of tests at 50, 70, and 90° F. In 
each case the room was maintained at the specified temperature 
for three hours before the tests were made. It will be noticed 
that the rubbers are not all affected to the same extent by equal 
differences in temperature, but there is a marked tendency in 
each case toward decreased strength, decreased set (increased 
elasticity), and increased elongation as the temperature is raised. 


















The Testing of Rubber Goods 


61 


It will be noted further that in nearly every case greater differ¬ 
ences occur between 50 and 70° than between 70 and 90°. 

The set in each case was measured after one-minute stretch and 
one-minute rest. Nos. 1 and 2 were stretched 350 per cent, Nos. 
3 and 4, 300 per cent, and No. 6. 250 per cent. 



TEMPERATURE 


3000 

2800 

2600 

2400 

2200 

2000 

1800 

1600 

1400 

1200 

1000 

800 

600 

400 

200 

0 


O' 

(/> 

or 

LU 

Q- 


CD 

CD 

_J 

I 

x 

h- 

O 

z 

UJ 

or 

h“ 

CO 


Pig. 32 .—Curve showing influence of temperature on the elasticity, tensile strength , and 

ultimate elongation of rubber 


(c) Influence of Cross Section on Tensile Strength and Ultimate 
Elongation. —Tensile strength and ultimate elongation are theo¬ 
retically independent of sectional area, but, as in other materials, 
there is a tendency for small test pieces to develop higher unit 
values than large ones. Complete data on this subject are not at 
hand, but it is thought that test pieces one-fourth and one-half 
inch wide will show, in general, but little difference in ultimate 











































62 


Circular of the Bureau of Standards 


elongation, but an appreciable difference in tensile strength, in 
the case of high-grade compounds. 

Comparative results obtained with test pieces one-fourth and 
one-half inch wide are shown in Table 3. These rubbers repre¬ 
sent commercial compounds. The test pieces were cut from the 
tubes and covers of plied hose, and the impression left by the 
fabric was carefully removed with the emery wheel shown in 
Fig. 18. 


TABLE 3.—Influence of Cross Section upon the Tensile Strength and Ultimate 

Elongation of Rubber 


M-4 cover. 
M-3 tube. 
G-19 tube. 
G-19 cover. 
M-5 tube.. 


Specimen 


Width of 
specimen 

Tensile 

strength 

Ultimate 

elonga¬ 

tion 

Inch 

Lb./in . 2 

Per cent 

f H 

1565 

525 

1 H 

1455 

515 

f H 

2160 

580 

1 

1955 

570 

f H 

1025 

350 

1 H 

955 

340 

* 

735 

255 

{ H 

690 

250 

J H 

2490 

615 

\ M 

2060 

575 


TABLE 4.—Influence of Direction in which Specimens Are Cut on Tensile Properties 


Sample No. 

Direction 

Tensile 
strength a 

Ultimate 

elonga¬ 

tion 

Set« 
alter 300 
per cent 
for 1 
minute 
with 1 
minute 
rest 



Lb.in . 2 

Per cent 

Per cent 

1 

| Longitudinal. 

2730 

630 

11.2 

1 . 

{Transverse.. 

2575 

640 

7.3 


[Longitudinal. 

2070 

640 

6.0 


{Transverse. 

2030 

670 

5.0 

3 

[Longitudinal. 

1200 

480 

22.1 


[Transverse. 

1260 

555 

16.3 

4 

(Longitudinal. 

1850 

410 

34.0 


[Transverse. 

1700 

460 

24.0 


[Longitudinal. 

6 QQ 



5 . 

[Transverse. 

M0 




[Longitudinal.. 

880 

320 

34.3 


(Transverse. 

690 

280 

25.9 


" The set and tensile strength were determined with different test pieces. 


(d) Influence of the Direction in which Specimens are Cut on 
Strength, Elongation , and Recovery. —The tensile properties of 
















































The Testing of Rubber Goods 


6 3 


sheet rubber are not the same in all directions, as will be shown 
later in connection with comparative tests of straight and ring- 
shaped test pieces. There is a tendencv for specimens cut longi¬ 
tudinally, or in the direction in which the rubber has been rolled 
through the calender, to show greater strength and (at least for the 
better grades of rubber) less elongation than specimens cut trans¬ 
versely or across the sheet. The recovery, however, is greater in 
the transverse direction. Table 4 illustrates this. 

No. 3 shows slightly greater strength transversely and No. 5 
greater elongation longitudinally, while No. 6 shows the same 
elongation in each direction. The exception noted in No. 3 is 
attributed to experimental errors or to small defects in the speci¬ 
mens which escaped detection. It may be, however, that low- 
grade rubbers, such as Nos. 5 and 6, are not more extensible 
transversely than longitudinally. Further tests are necessary to 
determine this point. 

(e) Influence of “ Backing ” on the Tensile Strength and “ Recov¬ 
ery ” of Hose Lining. —In the case of cotton rubber-lined hose the 
“backing” which is used to cement the lining to the fabric, if not 
removed, affects to a greater or less extent the values obtained 
for tensile strength and “set” or “recovery.” 

If the backing has an ultimate elongation greater than that of 
the lining, it must necessarily increase the tensile strength which 
is figured from the breaking load and the measured cross section 
of the rubber lining. The observed value for “set” which is 
obtained as described on p. 57, is greater or less than the true 
value, according as the lining is more or less elastic than the 
backing. If the elasticity of the backing differs very much 
from that of the lining, it will be found that the broken test pieces 
have a tendency to curl up like a watch spring, with the backing 
on the inside or outside according as its elasticity is greater or 
less than that of the lining. It is customary to test hose linings 
with the backing removed. 

In Table 5 are recorded the results of tests that were made to 
determine the effect of backing on the tensile properties of hose 
linings. For these tests the backing was removed with the emery 
wheel shown in Fig. 18. Differences in the values of tensile 
strength in lines A and B may be attributed partly to errors in 
measuring the thickness of the specimens that were tested without 
removing the backing. In the case of No. 1, however, it is 
thought that the thickness was determined without appreciable 
error. The adhesion between tube and backing being weak, it 


64 


Circular of the Bureau of Standards 


was possible to separate the two by hand, and the eight specimens 
thus secured showed a maximum variation in thickness of only 
o''.003. The average thickness was used in computing the 
strength of specimens tested with backing (line A). It is seen 
that in this case the backing very materially increased the strength 
of the test pieces. 

TABLE 5.—Effect of Backing on the Tensile Properties of Hose Linings 


Sample <* 


li A 
L B 
2, A 

2, B 

3, A 

3, B 

4, A 

4, B 

5, A 

5, B 

6, A 

6, B 

7, A 

7. B 

8, A 

8. B 


Set after 
elonga¬ 
tion and 
release 

Tensile 

strength 

Ultimate 

elonga¬ 

tion 

Per cent 

Lb./in. 1 2 3 4 5 6 

Per cent 

b 24.5 

3080 

590 

b 23.0 

2535 

575 

b 17.5 

3015 

625 

b 18.5 

2790 

635 

b 17.5 

3025 

645 

b 19.0 

2615 

610 

b 20.5 

2705 

650 

b 19.0 

2720 

645 

b 24.0 

2795 

580 

b 23.5 

2300 

585 

c 20. 5 

2555 

535 

e 22. 5 

2410 

540 

b 21.0 

2095 

550 

b 17.5 

2335 

590 

c 21. 5 

1190 

535 

c 20. 5 

1190 

535 


a A, tested without removing backing; B, tested with backing removed. 
b Measured after 400 per cent elongation for 10 minutes, with 10 minutes’ rest. 
c Measured after 350 per cent elongation for 10 minutes, with 10 minutes’ rest. 


(/) Influence of Previous Stretching on Strength, Elongation, 
and 11 Recovery." —Test pieces that have been used to determine 
set usually show greater strength and ultimate elongation than 
are obtained with test pieces that have not been previously 
stretched. 


TABLE 6.—Influence of Repeated Stretch on Tensile Strength and Ultimate 

Elongation 


Sample No. 


1 

2 

3 

4 

5 

6 


Tensile strength 

Ultimate elongation 

Single 
| stretch 

Repeated 

stretch 

Single 

stretch 

Repeated 

stretch 

Lb./in. 2 

Lb./in. 2 

Per cent 

Per cent 

2470 

2610 

645 

765 

1740 

1960 

665 

780 

990 

1180 

510 

645 

1710 

1790 

460 

555 

750 

790 

430 

440 

930 

920 

375 

465 


















































The Testing of Rubber Goods 


65 


Table 6 gives the strength and ultimate elongation obtained 
in testing six samples of rubber, first, with a single stretch, and 
second, by repeated stretching, beginning with 200 per cent and 
increasing each stretch by 100 per cent until failure. 

In testing rubber as described above, it is found that if during 
the latter part of the test the increments in extension for successive 
stretches are small (say, 25 per cent of the original length), a point 
is reached where the rubber breaks down, and beyond which it 
fails on a subsequent stretch at a tension less the maximum 
which it has already developed. 

The recovery after a definite elongation is usually greater if the 
rubber has been previously stretched than if determined in the 
usual way. This is illustrated by the results shown in Table 7, 
in which the columns marked “Repeated stretch” show' the set 
after repeated stretching, beginning with 100 per cent and increas¬ 
ing 100 per cent for each subsequent stretch. The results in 
columns marked “Single stretch” v r ere obtained in the usual w r ay, 
each specimen being stretched but once. In each case the set was 
measured from the original gage marks, after 1-minute stretch 
and 1-minute rest, the tabulated results being the average of 
the values obtained in testing a number of specimens. 

TABLE 7.—Influence of Repeated Stretch on the “Recovery” of Rubber 


Set (in per cent) after being stretched— 


No. 


(Repeated stretch 
[Single stretch . 
(Repeated stretch 
{Single stretch... 
3 (Repeated stretch. 
[Single stretch.. 
[Repeated stretch. 
[Single stretch... 
(Repeated stretch. 
[Single stretch.. 
(Repeated stretch. 
J [Single stretch... 


Method of testing— 


100 per 
cent 


200 per 
cent 


300 per 
cent 


400 per 
cent 


500 per 
cent 


1.0 4.5 


1.8 4.0 


3.7 9.0 


4.0 12.3 

14.3 
8.1 19.4 

_ 19.3 

4.3 16.3 

17.0 


9.5 

16.0 

25.0 

11.7 

19.8 

29.0 

7.7 

13.7 

21.2 

8.0 

14.7 

21.5 

17.7 

27.0 

37.0 

21.7 

34.0 

47.0 

28.7 

48.7 


33.0 

56.0 


34.0 



33.0 



34.0 


35.3 



It will be noted that the effect of previous stretching is very 
marked in the case of Nos. 1,3, and 4; that it is very slight in 
the case of Nos. 2 and 6; and that in the case of No. 5 the set is 
slightly increased by previous stretching. 

56597°—21-5 





































66 


Circular of the Bureau of Standards 


FTh 








1 \ ^ d' '/ 

\ \ 

\ \ / 

J'Jh 


(g) Influence of the Form of Test Piece on the Results of Tension 
Tests .—There is a wide difference of opinion in regard to the rela¬ 
tive merits of the straight and ring-shaped test pieces. The ring, 
which is highly recommended bv some, undoubtedly possesses cer¬ 
tain advantages as regards convenience in testing, and uniform 
results may be obtained by this method. 

Ring test pieces, however, do not show the full tensile strength 
of rubber, on account of the uneven distribution of stress over the 

cross section. This fact is 
evident from a simple 
analysis, and may be veri¬ 
fied by comparative tests 
with straight and ring- 
shaped test pieces, provided 
the straight test pieces are 
sufficiently enlarged at the 
ends to prevent failure in 
the grips, and provided 
further that the change in 
width is not made too 
abruptly. 

Assuming for simplicity 
that the extensibility of 
rubber is the same in all 
directions, it will be seen 
by reference to Fig. 33, 
which represents a ring 
test piece before and after extension, that, 

If E x =per cent elongation of the inside surface at break¬ 
ing point (automatically measured), 

then E { = — j —.100. 



BEFORE STRETCHING 


AFTER STRETCHING 

Fig. 33 .—Ring test piece before and after being 
stretched 


(l) 


If A 2 =per cent elongation of the outside surface when 
specimen breaks, 

then E 2 = — 5 .100 . 

L 2 =Li+ 7 t /2 (D, — Df) =Lj + 7 rT. 


T = t*lh- approximately. 

V -^1 

(assuming that the volume of rubber is constant.) 


(2) 

( 3 ) 

( 4 ) 






















The Testing of Rubber Goods 


67 


From equations 1, 2, 3, and 4 we have 

l£-u 


L x + irt 


\L X 


( 5 ) 


E,-eR . 

i 2 *1 

which is represented graphically in Fig. 34 (a) for the usual size of 
ring in which l x = 70 mm and t = 4 mm. 

This relation is practically a linear one, and E 2 =0.83 E x approxi¬ 
mately. Fig. 34 (6) shows the slight error introduced by neg¬ 
lecting the term 7 r/-»/ j- 



Now, since the percentage of elongation at the outside surface 
of the ring is less than at the inside surface, the tensile stress must 
also be less at the outside than at the inside surface. From equa¬ 
tion (5) it follows that the decrease in the percentage of elongation 
is approximately uniform from the inside to the outside of the 
ring, this relation being shown in Fig. 35 for E x = 600 per cent 
and l x = 70 mm. The relation between stress and elongation being 
practically a linear one for values of elongation near the breaking 
point, the decrease in tensile stress must also be fairly uniform 
from the inside to the outside of the ring at the time of failure. 
This is illustrated graphically in Fig. 35, in which the values for 
tensile stress were taken from the stress-strain diagram shown in 
Fig. 36, No. 1. 























68 


Circular of the Bureau of Standards 



t=distance from inside surface in mm 


Fig. 35 .—Variation in elongation and tensile strength from the inside to the outside of a 

ring 

If 5 ,= the stress at the inside surface of the ring at failure, or 
true tensile strength of the rubber corresponding to E lt and 
S 2 = the stress at the outside surface, corresponding to E 2 , and 
S = the average stress over the cross section of the ring, which is 
the value for tensile strength obtained by the ring method, we 
have 


^ Breaking load S { +S 2 
Area of section 2 



approximately 


( 6 ) 


Now, since the ratio varies for different rubbers, 5 does not 

bear a constant ratio to, and, therefore, can not be taken as, a 
measure of tensile strength. Elongation, however, is measured at 








































STRESS-LBS. PER SO. IN. STRESS—LBS. PER SQ. IN. 


The Testing of Rubber Goods 


69 




p IG — Stress-strain curves for rubber tested with straight and ring-shaped specimens 
















































































































jo Circular of the Bureau of Standards 

the inside surface of the ring and, therefore, represents the maxi¬ 
mum extension of the rubber around the inside of the ring. The 
average elongation over the cross section of the ring is, 

E = i >2 (E { +E 2 ) approximately.(7) 

If the extensibility of rubber were the same in all directions, 
values of 5 and E obtained from equations (6) and (7) would, 
theoretically, give a point lying very near the stress-strain curve 
for the same rubber tested in the form of a straight specimen. 
This, however, is not the case, as has been already pointed out, 
and as may be seen from Fig. 36. 

The difference between 5 . and S, is greater for high-grade 
rubbers than for compounds of poor quality, as may be seen by 
reference to Fig. 36, which represents stress-strain curves plotted 
from the results of tests on straight and ring specimens. Table 8 
shows values for tensile strength and ultimate elongation obtained 
for the same rubbers by the two methods. 

The ring test piece obviously does not give a true stress-strain 
curve on account of the varying stress over its cross section. 

TABLE 8.—Relative Tensile Strength and Elongation of Rubber Tested with Straight 

and Ring-Shaped Specimens 


Tensile strength, in pounds per square inch, for 
rubber compounds Nos. 1 to 6 


Specimens 


- 

1 

2 

3 

4 

5 

6 

Straight specimens: 







Longitudinal (L)«. 

2730 

2070 

1200 

1850 

690 

880 

Transverse (T). 

2575 

2030 

1260 

1700 

510 

690 

Ring specimens (R). 

2140 

1690 

1060 

1520 

510 

730 

R I.. . 

0.78 

0.82 

0.88 

0.82 

0. 74 

0.83 

R/T. 

0.83 

0.83 

0.84 

0.89 

1.00 

1.06 


Ultimate elongation, in per cent 


Straight specimens: 

Longitudinal. 

630 

640 

480 

410 

320 

315 

Transverse. 

640 

670 

555 

460 

280 

315 

Ring specimens. 

635 

675 

525 

435 

285 

320 


“ Longitudinal indicates the direction in which the rubber has been passed through the calender rolls. 


Straight specimens were cut both longitudinally and trans¬ 
versely, with reference to the direction in which the rubber had 
been passed through the calender rolls. They were tested with 
the machine shown in Fig. 24, and the ring specimens were tested 





































The Testing of Rubber Goods 71 

with a Schopper machine, Fig. 28. In each case the specimens 
were stretched at the rate of about 8 in. per minute. A number 
of test pieces, both straight and ring shaped, particularly in the 
case of No. 3, showed abnormally low tensile strength and elonga¬ 
tion on account of small holes or particles of grit at the point of 
rupture. Such specimens are not included in the results tabulated 
below, each of which represents the average of from 5 to 15 tests. 

A line was drawn across each of the ring specimens to indicate 
the longitudinal direction, and the point of failure was noted. 
There was a tendency for the rings to rupture along this line, thus 
indicating that the sheets were strongest longitudinally, or in the 
direction of rolling. This difference in strength is shown by the 
straight test pieces, except in the case of compound No. 3. It is 
seen from Fig. 36 that the curve for transverse specimens lies 
below that for longitudinal specimens, thus showing that a given 
stress will produce a greater elongation if applied transversely 
than if applied longitudinally. It is to be expected, therefore, 
that the elongation of a ring will be less than that for a transverse 
straight specimen. The natural variation in rubber, however, 
is often sufficient to obscure small differences in elongation, due 
to the methods of testing. 

In the case of Nos. 5 and 6, the curves for the ring specimens 
almost coincide with those for the transverse straight specimens, 
and the tensile strengths of these rubbers when tested by the two 
methods are seen to agree fairly well. It is to be noted, however, 
that for the higher-grade rubbers the difference in tensile strength 
by the two methods is very marked. Although the difference 
is not great, there is a tendency for the transverse specimens to 
show a greater ultimate elongation than the longitudinal specimens, 
notwithstanding the greater strength shown in the latter case. 

(6 ) “Friction” Test. —The “friction” or adhesion between 
the plies of fabric, or between the fabric and rubber parts, is of 
great importance; in fact, the life of pneumatic tires, belting, 
hose, etc., depends in a great measure upon the efficiency of this 
adhesion. For a detailed description of test methods see page 88. 

Friction is preferably determined autographically with the 
machine illustrated in Fig. 37, which records the tension required 
to cause a definite rate of separation between the two parts 
considered. 

The autographic machine for testing friction, Fig. 37, is operated 
by a one-eighth-horsepower worm-geared shunt motor which is 


72 


Circular of the Bureau of Standards 



Fig. 37 .—Autographic friction ma¬ 
chine for measuring and recording 
the adhesion between plies of fabric 


belted to a stepped pulley. A worm 
on the pulley shaft drives a worm 
wheel which is geared to a spur inside 
of the vertical steel column. This 
spur drives a steel rack to the upper 
end of which is attached the movable 
grip. From the top of the machine is 
suspended a spring which carries at its 
lower end a fork or suitable grip for 
holding the test piece. Between the 
spring and fork is a pencil holder in 
front of which is a drum carrying a 
paper chart on which the record is 
drawn. 

The capacity of the spring is 40 
pounds, the extension being 1 inch for 
10 pounds pull. The spring is replace¬ 
able, thus permitting the use of a spring 
suitable for the grade of material under 
test. The drum is driven by a cord 
which passes over a small guide pulley 
and thence to a spool on the spur gear 
shaft. The surface speed of the drum 
is the same as the rate of separation 
of the plies of fabric being tested. 

In the case of rubber hose, a i-inch 
section is fitted over a mandrel and 
placed in the fork suspended from the 
top of the machine, and the detached 
end of the fabric is secured to the lower 
grip as shown. 

In the case of rubber belting a i-inch 
strip containing two plies is used. The 
plies are separated for a short distance 
and the ends secured in the two grips, 
the upper grip being a clamp held in 
the fork above mentioned. 

The method formerly used, but which 
is rapidly going out of use, is illustrated 
in Fig. 38 and consists in measuring 
the rate of separation produced by the 
action of a dead-weight. 










The Testing of Rubber Goods 73 

A marked difference is often found in the friction between 
different plies of the same hose, as well as at different points along 
the same ply. Uniformity in the friction is desirable. 

The results of this test are influenced by the temperature con¬ 
ditions, the rate of stripping caused by a given weight being greater 
at high than at low temperatures. Also, the rate of stripping is 
greater if the mandrel fits snugly in the ring than if the ring is 
allowed to sag over a loose mandrel. The variation in friction, 
however, in the same hose is often such as to obscure these influ- 



Fig. 38 .—Apparatus for testing the “friction” of rubber hose by the dead-weight method 


ences, unless observations are made under conditions which differ 
greatly. 

In connection with this test, attention may be called to a point 
which, though generally recognized, is sometimes lost sight of in 
the interpretation of results. 

It has been observed that no stripping is produced by increasing 
the weight up to a certain point, after which the rate of stripping 
increases gradually at first, and then more rapidly, with small 
increments in weight, until finally a very small increase in weight 
causes a large change in the rate of stripping. The general 









74 


Circular of the Bureau of Standards 

behavior is illustrated graphically in Fig. 39, in which each point 
represents the average of a number of tests on a very uniformly 
frictioned hose. 

As a result of this behavior, an air hose, for example, which is 
required to show a rate of stripping not exceeding 6 inches in 10 
minutes under 25 pounds, might be regarded as of very inferior 
quality if it stripped, say, 20 inches in 10 minutes, whereas the 
same hose would probably show little or no stripping under 20 
pounds and come within the required limit under 22 pounds. 

In testing fire hose (see p. 89) the central portion of the lining 
is separated from the jacket for a short distance. The detached 
end of the jacket is clamped in a stationary grip, and the weight 
is suspended from the rubber lining. 



Fig. 39.— “Friction” test of rubber hose. Curves showing rate of stripping under dif¬ 
ferent loads 

The “friction” between the plies of duck in rubber belting is 
sometimes tested by applying the load in a direction at right 
angles to the plane of separation, as in the case of “plied” hose. 
As shown in Fig. 40, A, this is done by cutting the belt about 
halfway through along parallel lines 1 inch apart. The belt rests 
on horizontal supports just outside of the strip which has been 
cut, and the weight is suspended from the detached end of the 
duck. Fig. 40, B, illustrates a method by which a i-inch strip is 
held in a fixed clamp with the weight suspended from the end of 
a detached layer of duck. In testing bv this method it will be 
found that the angle of separation varies somewhat according to 
the thickness of the strip and that the results are thereby influ¬ 
enced to some extent. The difficulty may be avoided by subdi¬ 
viding the 1-inch section into strips of two plies each, as shown in 


































75 


The Testing of Rubber Goods 


I^ig. 40, C, and very satisfactory results are obtained in this way. 
It is found that for a given weight the rate of stripping is decid¬ 
edly greater by methods B and C than by method A. Fig. 41 
shows graphically the results obtained by the three methods. 

(7) Hydraulic-Pressure Test. —The pressure test as usually 
made consists simply in subjecting the hose to water pressure 
created by a force pump of any convenient type. The coupling 
at the free end is closed with a plug, and the pump connection 
is made with a reducing coupling. By using two clamps at each 



Fig. 40 .-—Illustrating methods of testing the “ friction” 

tion of dead weights 


of rubber belting by the applica- 


end it is possible to make a tight joint even under high pressure. 
It is necessary to provide a check valve to protect the pressure 
gage against shock when a hose bursts. A pet cock must be pro¬ 
vided to release the air as the hose is being filled. 

Requirements of specifications as regards the pressure test vary 
according to the kind of hose, but, as a rule, the test is made not 
with the view of developing the ultimate strength of the hose 
but rather to detect defects in workmanship, which are usually 
noticeable at a pressure well below that necessary to rupture the 
hose. 










































76 


Circular of the Bureau of Standards 

In the case of fire hose it is usual to specify a certain pressure 
when the hose is lying straight or when bent to the arc of a circle 
of given radius, and the hose must stand a specified pressure when 
doubled upon itself. A full 50-foot length must not show excessive 
expansion, elongation, warping, or twist under pressure, and the 
twist must be in a direction tending to tighten the couplings. 

(8) Steaming Test. —Fig. 42 illustrates a method of testing 
steam hose. The header 1 is provided with six outlets, each of 
which is controlled by a one-half-inch globe valve. The header 2, 
which is connected to a steam trap 3, is similarly provided with 



Curves showing rate of stripping when tested by the methods illustrated in Fig. 40 

inlets and controlling valves. The hose to be tested is cut into 
lengths that will just fit between the connections on the headers, 
the bottom connections being made with unions. Steam passes 
through a regulating valve (not shown) into the header i and 
thence through the hose to the header 2, from which the condensa¬ 
tion is carried to the steam trap. 

(9) Testing the Rubber Insueation of Wire. —The mechan¬ 
ical tests that are usually specified for the rubber insulation of 
wire are the same as those already described in connection with 
tension tests. 










































77 


The Testing of Rubber Goods 



The method of preparing test pieces, however, is not always the 
same, but depends upon the size of conductor and the character 
of insulation. (See p. 85.) 

(10) Comparative Tests of Machine and Handmade Tubes.— 
A question is sometimes raised as to whether the tube of a hose 
has been made in a tubing machine or from calendered sheet. 
Calendered tubes are often specified, and since it is sometimes 
impossible to determine by in¬ 
spection if a tube is machine- 
made or handmade, a test that 
could be depended upon in all 
cases to distinguish between the 
two kinds of tubes would serve a 
useful purpose. 

From the nature of the case a 
chemical analysis could not de¬ 
termine this point, because a com¬ 
pound might be calendered or run 
through a tubing machine with¬ 
out in any way altering its com¬ 
position. The mechanical prop¬ 
erties of a compound, however, 
are influenced to a greater or less 
extent bv the method of manu¬ 
facture. 

Comparative results for elastic¬ 
ity or “set,” tensile strength, 
and ultimate elongation are given 
in Table 9. Each of the three 
compounds was made into two 
tubes of the same size, one from 
calendered sheet and the other 
made in a tubing machine. The 
conditions of vulcanization were the same for each pair of tubes, 
so that any difference in their physical properties is fairly attrib¬ 
utable to the effects of the tubing machine and calender rolls. 

In comparing the results it is interesting to note that the 
squirted tubes show practically the same set when tested longi¬ 
tudinally and transversely, whereas the calendered stocks are 
much more elastic in the transverse direction, as has been pointed 
out in connection with tests previously described. This is more 
clearly illustrated in Fig. 43, which shows graphically the relation 


Fig. 42 .—Six samples of steam hose con¬ 
nected up for steaming test 









78 


Circular of the Bureau of Standards 


between set and elongation for test pieces cut longitudinally and 
transversely from the compound G-13. In the same way Fig. 44 
shows stress-strain curves for G-13. Similar tests of the other 
two compounds could not be made for lack of material. 


LEG£1/VD 


*- 

•- 

♦- 

i 


C -y/e rider’s c/ 7~ube. 
Sc?<j/rf<sc/ 7~c/b& 


( /ongr/ud nya/ / 
( frar?sv<grs<s ) 

C /orij/J-c/d/rro/J 

C■/■/-ons’ver&zs ) 

(Song arte/ Crarrs) 



E/ongof-/on ~ /=*&r c<ent 

Fig. 43. —Curves showing the elasticity of a “ squirted” tube as compared with a calen¬ 
dered tube, tested in the longitudinal and transverse directions 


TABLE 9 .—Relative Tensile Properties of “Squirted” and Calendered Tubes when 
Tested in the Longitudinal and Transverse Directions 


Sample No. 

Method of manufacture 

Set a after one-min¬ 
ute stretch and one- 
minute rest 

Tensile strength 

Ultimate elongation 

Longi¬ 

tudinal 

Trans¬ 

verse 

Longi¬ 

tudinal 

Trans¬ 

verse 

Longi¬ 

tudinal 

Trans¬ 

verse 



Per cent 

Per cent 

Lb./in. 2 

Lb./in. 2 

Per cent 

Per cent 

V 14. 

f Calendered.. 

11.7 

10.5 

520 

530 

275 

290 


(Squirted. . . 

12.5 

12.5 

460 

460 

240 

235 

V-16. 

(Calendered.... 

24.7 

20.0 

1450 

1380 

530 

535 


(Squirted. . 

23.0 

21.8 

1310 

1290 

525 

505 

0 n 

(Calendered -... 

26.0 

18.5 

1110 

1295 

455 

510 


(Squirted... 

22.5 

21.5 

1130 

1330 

470 

510 


“V 14 was stretched 200 per cent: V 16 and G-13, 35° per cent. 




















































The Testing of Rubber Goods 


79 


Definite conclusions should not be drawn from these prelim¬ 
inary tests, but the results, which indicate that the elastic proper¬ 
ties of squirted stock are practically the same in all directions for 
extensions up to at least 75 per cent of the ultimate elongation, 
may be verified by further experiments. 



Fig. 44.— Stress-strain curves for a “ squirted ” and a calendered tube tested in the longi¬ 
tudinal and transverse directions 


(11) Testing of Rubber Bands.—( a) Under One-fourth Inch 
in Width .—In the testing of rubber bands a different method is 
employed from that used in the testing of other types of rubber 
goods. Owing to the size of the specimens, it is impracticable 
to obtain accurately the cross-sectional area, consequently the 
tensile strength is not computed in pounds per square inch, as is 
customary in other goods. 

Double-spool grips similar to the grips illustrated in Fig. 23, B, 
but smaller, are used. The test machine used for this work is 
shown in Fig. 29, and a description may be found on page 56. 





























8o 


Circular of the Bureau of Standards 

Tensile Strength and Ultimate Elongation.—The tensile strength 
is measured in pounds and is expressed on the basis of two hundred 
or one hundred 2-inch bands per ounce, according as the bands 
are one-sixteenth or one-eighth inch wide. The elongation is 
expressed in percentages of the original length. 

The average inside length (measured flat) of several ounces of 
the bands to be tested is measured. These bands are weighed 
and the equivalent number of 2-inch bands per ounce (N) is 
calculated, the following formula being used: 


XL X 28.35 herg , 

2.0 X W 

n = number of bands weighed; 

L = average inside length of band in inches (measured flat); 
w = weight of bands in grams. 

The tensile strength on the basis of two hundred or one hundred 
2-inch bands per ounce (S) is calculated as follows: 

TN 

S = — for bands one-sixteenth inch wide; 

200 

TN 

S = -for bands one-eighth inch wide, where 

100 

T = tensile strength of bands in pounds. 


Ultimate elongation, per cent = 


(L — L t ) 100 


L x 


where: 


L= one-half the distance around spools at time of failure; 

Lj = initial inside length of band, measured flat (one-half total 
length). 


(6) Bands One-fourth Inch in Width or Over .—The testing of 
bands one-fourth inch in width or more requires the use of the 
grips shown in Fig. 23, B, and the tests are made on the machine 
usually employed for determining the tensile strength of rubber. 

(Fig. 24.) 

Tensile Strength and Ultimate Elongation.—In bands of this 
size it is practicable to measure the width and the thickness. 


1 If avoirdupois weights are used, the equation becomes 

N= 

2.0XW 


In this case w is the weight in ounces. 






The Testing of Rubber Goods 81 

The tensile strength is calculated in pounds per square inch as 
follows: 

Tensile strength in pounds per square inch *=— where 

B = breaking load in pounds; 
w = width of band in inches; 
t = thickness of band in inches. 


Elongation is measured and expressed in the same way as 
described above for bands under one-fourth inch in width. 

Jar rings are tested in the same way as rubber bands. Owing 
to the shape of the material the internal length L { becomes one- 
half of the internal circumference thus: 


.L x 


TV D 


, where 


D= internal diameter in inches. 


(b) BUREAU OF STANDARDS PROCEDURE FOR PHYSICAL TESTING OF RUBBER 

i. (i) Sampling. —Samples shall be taken directly from the 
finished material. They should be marked with the maker’s 
name, date of sampling, and sufficient other data to insure easy 
and complete identification. Unless otherwise specified, the size 
of sample should be in accordance with the following schedule: 


Amount of Material Required for Tests 

Hose of 3-inch diameter and less.2 feet. 

Hose over 3-inch diameter.1 foot. 

Hose for pressure or steam test.4 feet. 

Sheet packing.1 square foot. 

Insulated wire.4 to 6 feet. 

Bicycle and motor cycle tires.whole tire. 

Automobile tires, solid.section 30 inches long. 

Automobile tires, pneumatic..whole tire. 

Inner tubes.whole tube. 

Miscellaneous rubber goods, sufficient material to provide 

at least 15 pieces.1 by 6 inches. 


2 . ( 2 ) Physical Test Methods. — (a) Articles of Irregular 
Shape .—When articles are of such shape or size as not to admit 
of standard test pieces being prepared, the manufacturer shall 
submit test strips 8 inches long, 1X inches wide, and one-eighth 
inch thick, unless otherwise specified. The strips shall be guar¬ 
anteed to be of the same composition and cure as the articles 
delivered. 


56597°—21-6 















82 


Circular of the Bureau of Standards 


3. (b) Preparation of Samples for Test .—Separating Rubber 
From Fabric.—The gasoline used in separating rubber from fabric 
should be 72 to 76° Be, and upon evaporation should not leave 
an appreciable amount of oily residue. To avoid stretching the 
rubber unnecessarily it is desirable to cut the material into strips 
slightly wider than a test piece, and the separation should be 
made gradually and a little at a time, while the rubber is gripped 
near the point of separation. The rubber should then be so 
placed as to permit free evaporation from all parts of its surface. 
Benzene (benzol) can also be used. 

4. Removing Fabric Impressions.—Any unevenness of surface, 
such as impressions caused by contact with fabric parts, which 
would interfere with an accurate measurement of thickness, is 
removed by careful grinding with an abrasive wheel of about 
No. 30 grit. (See Fig. 18.) A wheel for this purpose should be 
provided with a slow feed in order that very little rubber be 
removed at one cut, otherwise the rubber may be injured by 
overheating. The speed should be from 2500 to 3000 rpm for a 
wheel of 5 inches diameter. If a backing is used, as in the case 
of cotton rubber-lined fire hose, it should be entirely removed in 
the same way. The face of the wheel should be kept sharp. 

5. Cutting Test Pieces for Determinations of Tensile Strength, 
Ultimate Elongation, and Set.—Test pieces shall be cut with a 
metal die (see Figs. 20 and 21) which should be kept sharp to 
avoid leaving ragged edges on the rubber. 

6. The distance between cutting edges of the blade over that 
portion of the die corresponding to the gage length of the test 
piece shall not vary more than 0.002 inch. 

7. An arbor press (see Fig. 19) is recommended for forcing the 
die through the rubber in preference to the practice of striking 
the die with a mallet. The operation is facilitated by wetting 
the cutting edges of the die. The rubber should rest on a smooth 
and slightly yielding surface which will not injure the blade. A 
piece of rubber belting, or preferably leather belting, is suitable 
for the purpose. When the material on which the test pieces 
are cut has become slightly rough from use, a sheet of paper 
placed under the rubber will be of advantage. 

8. Shape and Size of Test Pieces for Determinations of Tensile 
Strength, Ultimate Elongation, and Set.—Unless otherwise speci¬ 
fied, the central portion of test pieces shall be one-fourth inch wide 
over a gage length of 2 inches, the ends being gradually enlarged 
to a width of 1 inch to provide a satisfactory gripping surface. If 


The Testing of Rubber Goods 83 

one-half-ineh test pieces are specified the end portions should have 
a width of 1X inches. (See Fig. 20, A.) 

9. In the case of pure gum or compounds containing a large 
percentage of rubber which have a very great elongation, shorter 
test pieces one-fourth inch wide over a gage length of 1 inch and 
\ X / A inches wide at the ends are used. (See Fig. 20, C.) 

10. In the case of insulated wire and cable the form of test piece 
shall be as described in paragraph 22, entitled “Insulated Wire.’’ 

11. Gage Marks.—Unless otherwise specified, the gage length 
shall be 2 inches, except that if the rubber has an elongation greater 
than 700 per cent, or if the article to be tested is not of sufficient 
size to produce a test piece of this length, the gage length shall be 
1 inch. Gage marks shall be made with ink by using a stamp con¬ 
sisting of parallel steel blades which produce very fine lines, care 
being taken in all cases not to injure the rubber. 

12. Temperature of Testing Room.—All tests of rubber shall be 
made at a room temperature between 65 and 90° F. The samples 
should remain at this temperature for at least one hour before 
being tested. 

13. (d) Preparation of Tension Test Pieces .—Unless otherwise 
specified eight test pieces shall be prepared, four for the tensile 
strength test and four for the set test. 

14. Sheet Packing, Rubber Belting, etc.—Test pieces from mate¬ 
rial of this sort may be cut in any direction. Strips are first cut 
of a length and width slightly greater than the corresponding 
maximum dimensions of the test pieces. If the material contains 
fabric, the rubber is carefully separated as described in paragraph 3, 
and after the gasoline has evaporated the surface is buffed over 
the central portion of the strip for a distance somewhat greater 
than the gage length of the test piece. The test piece is then cut 
out with a die as described in paragraph 5. 

15. Hose.—Unless otherwise specified, as in the case of cotton 
rubber-lined fire hose, test pieces shall be cut longitudinally. 

16. In the case of plied hose, sections of sufficient length for 
test pieces are cut out. The sections are cut into longitudinal 
strips slightly wider than the test pieces, after which the cover 
and the tube are separated from the fabric as described in para¬ 
graph 3. When the gasoline has evaporated, the fabric impres¬ 
sions are removed by buffing over a portion of the strip slightly 
greater than the gage length, as described in paragraph 4. The 
test pieces are then cut out with a die as described in paragraph 5. 


8 4 


Circular of the Bureau of Standards 


When the diameter of the hose is one-half inch or less, test pieces 
should be first cut with a die and then buffed. 

17. With cotton rubber-lined hose, a section of sufficient length 
to produce the desired number of test pieces is cut. This section 
is cut at the lap and subdivided into transverse strips from which 
the cotton jacket is removed by the use of gasoline. When the 
gasoline has evaporated, the rubber backing is buffed off and the 
test pieces are cut out as described above, except that the central 
portion of the test piece shall be one-half inch wide over a gage 
length of 2 inches. (See Fig. 20, A.) The fold shall be within 
the gage length. 

18. To determine the strength of the lap, another section of 
sufficient length to produce the desired number of test pieces 
is cut. This section is cut so that the lap will be in the center of 
the constricted portion of the test piece. The cotton jacket is 
removed by the use of gasoline. When the gasoline has evaporated 
test pieces are cut out as described above, without being buffed. 

19. Solid Rubber Tires.—First cut from the tire a section of 
rubber 6 inches long, measured in the direction of its circumfer¬ 
ence. From the center of this section cut a longitudinal strip 
\]/ 2 inches wide normal to the axis of the tire. This strip is sub¬ 
divided in a slicing machine or by other suitable means into 
strips 1 )/ 2 inches wide and one-eighth inch thick. These strips are 
buffed if necessary before being cut into test pieces. 

20. Pneumatic Tires.—Strips 6 inches long and 1inches wide 
are cut longitudinally from the center of the tread and from the 
side wall, and the rubber is separated from the fabric with the aid 
of gasoline. The nonskid portion of the tread rubber is sliced off 
with a knife, after which the central portion is buffed on both 
sides over a length of 2 }/ 2 inches until free from all friction com¬ 
pound, fabric impressions, or any irregularities of surface. The 
side wall is buffed on one or both sides, as may be necessary. 
The test pieces shall be of the shape and size illustrated in Fig. 
20, B. 

21. Inner Tubes.—Strips 5 inches long and inches wide 
are cut longitudinally from the tube. These strips shall not be 
buffed unless the impressions left by the wrapping fabric are so 
pronounced as to vitiate the results of tests. The die used for 
cutting test pieces from this grade of rubber should be kept very 
sharp and each strip of rubber should be thoroughly wet before 
the die is used. A piece of paper should be inserted under the 


The Testing of Rubber Goods 85 

strip before the test piece is cut. The test pieces shall be of the 
shape and size illustrated in Fig. 20, C. 

22. Insulated Wire.—When the diameter of wire is large enough 
and the insulation thick enough, test pieces are prepared as fol¬ 
lows: The insulation is cut through to the wire in the longitu¬ 
dinal direction and the rubber is removed in one piece. The rub¬ 
ber is then laid out flat and cut with a die. Test pieces are buffed 
on both sides until they are smooth all the way across the con¬ 
stricted portion. 

23. When the diameter of wire is too small or the insulation 
too thin to permit of buffing, test pieces may be cut to better 
advantage if the rubber is held down with pins. In this case the 
cross section of test piece is approximately a sector of a ring. 

24. When the diameter of the wire is too small to permit of 
specimens being cut with a die, the insulation is tested as a whole. 
The general method for removing the insulation as a whole from 
a single wire is as follows: The insulation is removed for a distance 
of one-half inch at each end of a 7-inch length of wire and a slight 
nick is made in the wire at one end of the insulation. The un¬ 
covered ends are gripped in the jaws of a testing machine and the 
wire pulled until it breaks at the nick. The stretching of the wire 
reduces its cross section, and thus facilitates the removal of the 
insulation. The end of the wire should be rounded before pulling 
it through the insulation as the rough end might cut the rubber. 
In some instances adhesion between the wire and rubber is so 
great that the insulation can not be easily removed after the above 
treatment. When this is the case the sample is immersed in 
mercury, which forms an amalgam with the tin coating on the 
wire. Within one-half hour the amalgamation has usually prog¬ 
ressed sufficiently to permit the insulation to be easily removed. 
When there is a longitudinal bead or fin on the surface of the 
insulation it should be removed with a small sharp wood plane 
before the wire is withdrawn. 

25. (e) Tensile Strength and Ultimate Elongation. —Measure¬ 
ment of Cross Section.—The width of the test piece shall be 
measured with a gage or caliper (see Fig. 22) graduated to 0.001 
inch, the instrument being used in such a way that the compres¬ 
sion of the rubber between the contact surfaces is negligible. 
The thickness of the test piece shall be measured with a gage 
graduated to 0.001 inch, under a pressure of approximately 3 
ounces, exerted by a contact foot of approximately 0.3 inch dia¬ 
meter. The cross section shall be considered as the product of 


86 


Circular of the Bureau of Standards 


the minimum width by the minimum thickness of the test piece 
between the gage marks. 

26. If the cross section of the test piece is not of rectangular 
shape, as in the case of wire insulation, its area is computed from 
measurements as follows (Fig. 45): 

(1) When the test piece is cut with a die and buffed (par. 22) 
the width is taken as the average of the top and bottom widths 
of the buffed portion between the gage marks. 

(2) When the test piece is cut with a die and not buffed (par. 23) 

o j-p 

A = -~(D + d){D-d), where 

A = area of cross section in square inches; 

D = outside diameter of insulation in inches; 
d = diameter of wire or wire core in inches; 

Arc = width of test piece corresponding to outside diameter D. 



Died out and buffed Died oaf and naf buffed 


Whole section 


Fig. 45 .—Cross section of test pieces 


(3) When the insulation is tested as a whole, in the form of a 
hollow cylinder (par. 24) 

A = 0.7854 (D + d) (D — d ), where 
A = area of cross section in square inches; 

D = outside diameter of insulation in inches; 
d = diameter of wire or wire core in inches. 

27. Testing Machine.—The machine (see Figs. 24, 25, 26, 27, 
28, and 29) used for this test shall fulfill the following requirements: 

(1) The dial or scale for indicating the applied tension shall be 
accurate within o. 1 pound for samples having a breaking strength 
less than 15 pounds. For all loads above 15 pounds the maximum 
error shall not exceed 0.5 pound. The indicator shall remain at 
the point of maximum load after rupture of the test piece. 

(2) The grips (see Fig. 23) for holding test pieces shall be of such 
a design that a uniform pressure will be exerted across the gripping 
surface to avoid uneven slipping, and to insure failure of the test 
piece within its constricted portion. (3) The machine shall be 
power driven, and the rate of separation of the grips shall be 20 
inches per minute, unless otherwise specified. 




The Testing of Rubber Goods 


87 


28. Method of Test.—Care should be exercised to adjust the 
test pieces symmetrically in the grips in order that the tension 
shall be distributed uniformly over its cross section. If the tension 
is greater on one side of the test piece than on the other, the gage 
marks will not remain parallel and the maximum strength of the 
rubber will not be developed. The ultimate elongation may be 
measured by holding a scale (graduated to 0.1 inch) in a slightly 
inclined position at the back of the test piece with its lower end 
pressed lightly against the rubber just back of the lower gage mark. 
As the rubber is stretched the eye follows the scale just back of 
the upper gage mark. With a little practice the elongation at 
break may be measured with an error not exceeding 0.05 inch. 

29. Computation of Results.—Tensile strength is expressed in 
pounds per square inch, and is determined by dividing the break¬ 
ing load in pounds by the minimum cross section of the test piece 
in square inches. Ultimate elongation is expressed in percentage. 
For example, if a test piece of cross section 0.25 by 0.1 inch 
failed at a tension of 50 pounds, when the distance between the 
gage marks was 14 inches, the tensile strength would be 50^ (0.25 
X0.1) = 2000 lbs./in 2 , and the ultimate elongation would be 14-2 = 

12 inches, or 600 per cent. 

30. (/) Set. —Definition.—The term “set” refers to the exten¬ 
sion remaining, after a specified elongation for a given period of 
time, followed by a specified interval of rest. 

31. Testing Machine.—The machine recommended for this test 
is illustrated in Fig. 30. This machine is adapted to test six 
specimens at once, thus permitting a very great saving of time. 

32. Method of Test.—The test piece is stretched at an approx¬ 
imately uniform rate of speed such as to require about 15 seconds 
to reach the specified elongation and is held in this position for 
10 minutes, including the time required for stretching (unless 
otherwise specified), after which it is immediately released (with¬ 
out being allowed to snap back), and laid out on the test table. 
After a rest of 10 minutes (unless otherwise specified), the dis¬ 
tance between the gage marks is measured to the nearest 0.01 
inch and the set recorded in percentage of the original gage 
length. In stretching a test piece it is convenient to use a meas¬ 
uring rod of a length equal to the exact distance required between 
the gage marks. By holding the rod beside the test piece while 
it is being stretched the operation is simplified and the chance of 
stretching the test piece more or less than the desired amount is 


88 


Circular of the Bureau of Standards 


very much reduced. In taking the time qf the various operations 
in this test, a stop watch or a watch having a second hand should 
be used. 

33. Computation of Results.—For example, a test piece is 
stretched from 2 to 10 inches (400 per cent) for 10 minutes and 
then released. Its length measured after 10 minutes’ rest is 2.4 
inches, so that the set under these conditions is 0.4 inch, or 20 
per cent. 

34. ( g ) Friction. —Definition.—The term “friction” is used to 
indicate the adhesion between the two parts considered and is 
expressed numerically by (1) the average tension, in pounds, 
required to cause separation at a definite rate (1 inch per minute, 
unless otherwise specified) under stated conditions, or (2) the 
average rate of separation in inches per minute caused by a spec¬ 
ified tension exerted between the parts under the stated condi¬ 
tions. In either case, the test piece is 1 inch wide unless otherwise 
specified. 

35. Preparation of Test Pieces.—Wrapped Hose.—The most 
satisfactory way of preparing test pieces is as follows: A short 
length of hose is pressed snugly over a smooth, slightly tapered 
mandrel. The mandrel is put in a lathe, and 1 -inch sections or 
rings are cut with a short pointed knife, which is forced grad¬ 
ually through the cover and plies of fabric, but not entirely 
through the rubber tube. Upon removal from the mandrel the 
rings are easily cut apart with a knife The pointed knife used 
in cutting the rings should be kept sharp and should be wetted 
before each cut. A cut is made through the rubber cover at the 
point where the outside ply of fabric ends. The fabric is then 
unwound until the outside ply carrying the rubber cover is en¬ 
tirely separated from the rest of the sample. 

36. Braided Hose.—One-inch rings are cut as for wrapped hose. 
A cut is made through the rubber cover and first ply of braid. 
The braid is stripped just enough to admit of a clamp being 
attached. 

37. Hose Reinforced with Metal.—When metal reinforcement 
is present as in suction hose, which is reinforced by means of 
metal embedded in the rubber tube, between the plies of duck, 
or embedded in a layer of rubber in the central portion of the 
hose, test pieces are prepared as follows: A transverse section 
2 IT inches long is cut from the hose and opened so as to form a flat 
strip. Two parallel lines 1 inch apart are drawn on the rubber 


8 9 


The Testing o) Rubber Goods 

tube, the reinforcement being symmetrically located between 
them. The sample is then cut through on these lines making a 
strip i inch wide and equal in length to the circumference of the 
hose. 

38. Cotton Rubber-Lined Hose.—With a soft pencil draw T two 
parallel lines 2 }/ 2 inches apart, following the filler strands around the 
circumference of the hose. Cut the hose at the lap and also along 
these lines so as to form a strip of a length equal to the circum¬ 
ference of the hose. This strip is laid out flat, and the rubber 
lining cut through to the jacket along parallel lines 1 ]/ 2 inches apart 
or as may be specified. The central portion of the lining between 
these two cuts is separated from the jacket for a short distance. 

39. Belting.—A i-inch section about 8 inches long is cut from 
the belt in the longitudinal direction unless otherwise specified. 
This section is subdivided into strips of tw r o plies each, one end 
of which is separated for a distance of 2 inches. 

40. Packing.—Test strips are prepared from sheet packing, 
and from square packing in the same way as from belting, except 
that if square packing is of a size that does not permit of a i-inch 
strip a narrower test piece shall be used. Test pieces are pre¬ 
pared from round packing in the same manner as from plied hose. 

41. Pneumatic Tires.—Friction is determined with a i-inch 
section of the tire. The test piece may be conveniently prepared 
as follows: Cut a section approximately 2 inches long and remove 
the beads. Wrap the section tightly around a mandrel, using 
friction tape. The mandrel is put in a lathe and a i-inch section 
cut with a pointed knife. 

42. Testing Machine.—Friction should preferably be deter¬ 
mined with a machine (see Fig. 37) which automatically records 
on a chart the value of the friction at all points of the test piece. 
The machine should be adapted to maintain a uniform rate of 
separation of the parts as specified. 

43. Method of Test.—If so specified, or if the nature, form, or 
construction of the material is such as to preclude the use of a 
testing machine, the dead-weight method is used. This consists 
in suspending the specified weight from the detached end of one 
of the parts to be separated. The length stripped is measured 
after the weight has been removed, between marks made on the 
other part at the beginning and end of the test. The duration 
of the test is timed with a stop watch or a watch having a second 
hand. 


90 


Circular of the Bureau of Standards 


44. Wrapped Hose.—A test piece prepared as in paragraph 35 
is pressed snugly over a short wooden mandrel which is free to 
revolve in roller bearings attached to the upper jaw of the testing 
machine. The detached end of the fabric is held in the lower 
jaw. The downward movement of the lower jaw produces a 
radial pull which separates the fabric, and at the same time 
makes a graphical record which shows the length of fabric sepa¬ 
rated and the tension required at each point. In this case the 
rate of separation of the jaws is the same as the rate of stripping. 

45. Braided Hose.—The method is the same as for w r rapped 
hose. 

46. Hose Reinforced with Metal.—The method is the same as 
that for belting except that the test is made upon the entire 
section of hose. 

47. Cotton Rubber-Lined Hose.—The end of the fabric from 
which the rubber has been separated is clamped in a stationary 
grip. From the detached end of the rubber is suspended a 
weight of 12 pounds, and the rate of separation is determined 
from the length stripped during a measured interval of time. 
Measurements are made to the nearest 0.05 inch, between marks 
made on the fabric at the beginning and end of the test. The 
time shall be measured with a stop watch or with a watch having 
a second hand, and the duration of test shall be five minutes 
or such part thereof (measured to the nearest second) as may be 
required to strip a length of 5 inches. 

48. Belting.—One of the loose ends of a test strip (prepared as 
in par. 39) is clamped in the upper jaw and the other end in the 
lower jaw of the testing machine. In this case the rate of separa¬ 
tion of the jaw r s is twice the rate of stripping. 

49. Packing.—Method is same as for belting. 

*50. Pneumatic Tires.—Between tread and breaker: Remove 
the tread at one end up to the breaker strip and separate the two 
for a distance of about one-fourth inch. The detached end of the 
tread is clamped in the lower jaw and the corresponding end of 
the tire section in the upper jaw. 

51. Between side wall and carcass: When the friction test 
between tread and breaker strip has been made, remove the test 
piece from the machine and separate the tread by hand until the 
point has been reached where the side wall joins the carcass. 
Cut off a portion of the tread, leaving only sufficient material to be 
clamped in the lower jaw; clamp the corresponding end of the 
tire section in the upper jaw. 


The Testing of Rubber Goods 91 

52. Between breaker and cushion: Remove the tread (with 
breaker strip attached) for a sufficient distance to admit of its 
being clamped in the lower jaw; clamp the corresponding end of 
the tire section in the upper jaw. 

53. Between cushion and carcass: Remove the cushion (with 
breaker strip attached) for a sufficient distance to admit of its 
being clamped in the lower jaw; clamp the corresponding end of 
the tire section in the upper jaw. 

54. Between plies of fabric: The tread, breaker strip, and 
cushion having been removed, the outside ply of fabric is sepa¬ 
rated for a sufficient distance to admit of its being clamped in the 
lower jaw. The corresponding end of the tire section is clamped 
in the upper jaw. The other plies of fabric are separated in suc¬ 
cession and tests are made in the same way. Cord tires require 
special treatment because it is necessary to clamp at least three 
plies of fabric in each jaw. 

55. If during a test one of the parts begins to tear, instead of 
separating from the other part the material being torn is cut with 
a knife up to the surface of contact between the two parts and the 
test started again. 

56. The graphical record shows the length stripped and the 
tension required at each point. 

57. Computation of Results.—The value of the friction is 
recorded as the average tension required to cause separation at 
the rate specified, this being determined over that portion of the 
chart corresponding to an actual separation of the parts being 
tested. If one of the parts repeatedly tears, instead of separating 
from the other part, the average load at which tearing takes 
place is taken as the value of friction. 

58. If the dead-weight method is used, the value of friction 
is recorded as the average rate of separation under the action of a 
specified weight. 

59. (h) Hardness. —Definition.—The term “hardness” is used 
to express the depth of indentation produced by a spherical 
ended plunger of definite size, under a stated normal pressure 
exerted for a period of one minute. 

60. Instrument for Measuring Hardness.—There appears to 
be no definite relation between the indications of different instru¬ 
ments that have been designed for measuring the hardness of 
rubber. 

61. Unless otherwise specified the instrument used will con¬ 
sist of a plunger with spherical end of 3.2 or 6.4 mm diameter on 


92 


Circular of the Bureau of Standards 


which a pressure of i kilogram is exerted by means of a dead¬ 
weight and a gage graduated to one one-hundredth millimeters 
for measuring the depth of indentation. The smaller plunger is 
used except in the case of very soft rubbers. 

62. Method of Test.—The sample to be tested is supported 
in a horizontal position and the instrument is adjusted so that the 
plunger is vertical. In testing sheet rubber the sample should 
rest on a smooth unyielding surface. 

63. The needle of the gage is set at zero with the plunger rest¬ 
ing on the sample, after which the 1-kilogram weight is lowered 
upon the plunger and allowed to remain for one minute. The 
reading on the gage dial then shows the depth of indentation in 
millimeters. 

64. The average of four readings is recorded as the hardness of 
the sample. 

65. (i) Steam Test. —Hose.—Unless otherwise specified, steam 
hose is tested as illustrated in Fig. 42. The header 1 is provided 
with six outlets, each of which is controlled by a one-half-inch 
globe valve. The header 2, which is connected to a steam trap 3, 
is similarly provided with inlets and controlling valves. The hose 
to be tested is cut into lengths that will just fit between the con¬ 
nections on the headers, the bottom connections being made with 
unions. Steam passes through a regulating valve (not shown) into 
header 1 and thence through the hose to the header 2, from which 
the condensation is carried to the steam trap. 

66. The hose is subjected in this way to the action of steam at 
a pressure of 50 lbs./in. 2 for six days of seven hours each, the steam 
being turned off and the hose allowed to cool during the intervals. 

67. The results of tension and friction tests before and after 
steaming are indicative of the quality of the hose. 

68 . Steam Packing.—-Sheet packing is bolted between iron plates 
and subjected to the action of steam in an autoclave as illustrated 
in Fig. 46. Steam is generated by an automatic gas burner which 
is controlled by the pressure gage, the arrangement being such 
that the needle on the gage opens and closes the gas valve by 
making electrical contact at points corresponding to the minimum 
and maximum pressures for which the contact points are set. 

69. The test samples, several of which may be steamed at once 
by placing thin sheets of iron between them, are supported on a 
wire stand which holds them well above the water level. 

70. Valves.—Rubber pump valves are steamed in the autoclave 
as described above, except that they are not bolted between plates. 


The Testing of Rubber Goods 93 

7 1 * (j) Hydraulic-Pressure Test. —Fire hose.—A 3-foot sample 
when subjected to hydraulic pressure increasing gradually at the 
approximate rate of 3°° pounds per minute should not burst at a 
pressure less than that specified. The test is made first with the 
hose lying straight, and second with the hose held to a circular arc 
of 27 inches radius. 1 he coupling remote from the source of water 
is closed with a cap or plug provided with a pet cock to permit the 
escape of air while hose is being filled with water. 



Fig. 46.— Autoclave/or testing valves and packing in steam under high pressure 

72. A full length of hose is laid out straight on a smooth sur¬ 
face such as a cement walk. One coupling is connected to the 
source of water supply, the other coupling being closed with a cap 
or plug provided with a pet cock for the escape of air while the 
hose is being filled with water. To insure the complete removal 
of air from the hose, the surface on which the hose rests should be 
slightly inclined toward the source of water supply. 




94 


Circular of the Bureau of Standards 


73. With a crayon or soft pencil three marks are made around the 
hose jacket, dividing its length approximately into four equal 
parts. The circumference of the hose is measured at these marks 
during the test, as described later. 

74. With the pet cock open, admit water into the hose gradually 
until the air has been expelled and the hose is completely filled 
with water. The pet cock is then closed and the pressure gradu¬ 
ally increased until the gage (which has been tested for accuracy) 
shows a pressure of 10 lbs./in. 2 , when the water supply is cut off 
and a mark made on the top surface of the hose jacket adjacent 
to the closed coupling. This mark is used as a means of measuring 
the amount of twist during the test. 

75. With a steel tape measure the length of hose between backs 
of couplings, recording the result to the nearest one-quarter inch, 
and with a small flexible steel tape measure the circumference of 
the hose at the three equidistant points above referred to and 
record the results to the nearest one-thirty-second inch. 

76. Water is now gradually admitted into the hose in such 
quantity as will increase the pressure per square inch at an approxi¬ 
mate rate of 300 pounds per minute, and while the pressure is being 
increased the hose is carefully examined for leakage or other defects. 
When the test gage indicates the pressure specified for the kind of 
hose being tested, the source of water supply is shut off and the 
hose allowed to remain under this pressure for 10 minutes. When 
8 minutes have elapsed, the following measurements are taken and 
recorded: (1) The length of hose between the backs of couplings, 
following the contour of the hose; (2) the circumference of hose at 
the three equidistant points; (3) the amount of twist as indicated 
by the mark on the hose jacket adjacent to the closed coupling; 
(4) the amount of “warp” or deviation from a straight line as 
measured from a cord stretched from center to center of the backs 
of couplings; and (5) the rise from the surface on which the hose 
rests. 

77. Two pressure gages should be provided for this test, one 
graduated from o to 20 lbs./in. 2 and the other from o to about 
1000 lbs./in. 2 The low-pressure gage is used only to indicate the 
pressure of 10 lbs./in. 2 , at which the initial measurements are 
taken. It should be provided with a shut-off valve and relief 
cock to protect it against the higher pressures used in the test. 

78. ( k ) Interpretation of Results .—The average of the results of 
four test pieces is used for a determination of tensile strength, 


The Testing of Rubber Goods 


95 


ultimate elongation, and set. If in any case the result of a single 
test piece is found to be very much lower than that of the others, 
indicating a flaw in the material, the low result is discarded and 
the average of the results of the other three tests is recorded. If 
the result thus obtained fails by less than 5 per cent to meet the 
requirements of the specifications, a check test is made with four 
additional test pieces, the results of which, computed as above, 
shall be considered final. 

79. The value of friction is based on the result of a single test. 
If the average value as determined with an autographic machine 
fails by less than one-half pound to meet the specification, or if 
the result of a dead-weight test is greater than 1 inch per minute, 
but less than 1.1 inches per minute, a check test is made, the result 
of which shall be considered final. 

2 . BUREAU OF STANDARDS METHODS OF CHEMICAL ANALYSIS 

The methods given below are not entirely original, but have 
been compiled from the various publications on rubber analysis, 
from the information gained through the routine testing of rubber 
goods for delivery on Government contracts, and from coopera¬ 
tive research with various scientific organizations. These methods 
are subject to revision whenever this Bureau is convinced that 
changes would improve them. 

(a) REASONS FOR THE ANALYSIS 

Acetone Extract. —If the acetone extraction is made on a 
vulcanized compound, the acetone extracts the rubber resins, 
the free sulphur, any mineral oils or waxes, and part of any bitu¬ 
minous substances or vulcanized oils that may have been used. 
The percentage of free sulphur is determined and deducted from 
the total extract. The corrected figure thus obtained will at times 
give valuable information regarding the quality of the rubber 
present. For the best grades of Hevea rubber this should not 
exceed 5 per cent of the rubber present. A higher extract may 
indicate the presence of inferior or reclaimed rubbers. If the 
acetone extract solution is fluorescent it indicates the presence 
of mineral oil. 

Free Sulphur. —The free sulphur is that part of the sulphur 
originally added as such which remains unchanged after vulcani¬ 
zation. Small amounts of free sulphur are not harmful, but 
there are some who object to it in excessive amounts, claiming 
that in such cases it increases the rate of deterioration of the rub- 


96 


Circular of the Bureau of Standards 


ber. It is difficult, however, to place a limit beyond which the 
free sulphur is to be considered excessive. 

A limit is usually placed on the free sulphur in high-grade insu¬ 
lation compounds, not particularly on account of its effect on 
the rubber, but because it may corrode the copper wire. 

Total Sulphur. —Sulphur occurs in vulcanized rubber as 
free sulphur, in combination with the rubber, and at times in 
the mineral fillers and rubber substitutes. 

It is limited by specification in high-grade material in order to 
eliminate undesirable sulphur minerals and to prevent as far as 
possible the use of inferior or reclaimed rubbers and rubber sub¬ 
stitutes. The inferior rubbers require a larger percentage of sul¬ 
phur than Hevea rubber for proper vulcanization, while the 
reclaimed rubber and substitutes usually contain large amounts of 
sulphur. If it is desired to have material made from Hevea rub¬ 
ber only, the effect of placing the limit for sulphur where it will 
just suffice for the vulcanization of the amount of Hevea rubber 
called for will be to make it difficult to use inferior grades of rubber, 
reclaimed rubber, and substitutes and still have the total sulphur 
fall within the specified limit. 

Ash and Sulphur in the Ash.— The ash is the residue left 
after ignition. It consists principally of the nonvolatile mineral 
fillers. The percentage of ash is of no great importance in itself, 
but is used in the calculation of the rubber present, as will be 
explained later. 

The sulphur in the ash consists of the sulphur from some of the 
mineral fillers and also part of the sulphur that was combined 
with the rubber, but which during ignition enters into combina¬ 
tion with one or another of the mineral fillers. Its amount is de¬ 
termined merely for the purpose of obtaining a correction figure 
and has no other significance. 

Barytes. —It has been stated above and elsewhere that sulphur 
may be present in the mineral fillers. There can be no objection 
to such sulphur provided the mineral containing it has no inju¬ 
rious effect on the rubber and, further, that the amount of such 
sulphur can be readily determined. Barytes is such a substance, 
and it is permitted in practically all compounds where the amount 
of sulphur is limited by specifications. Barium carbonate is some¬ 
times used, and this must be considered when correcting for 
sulphur as barytes. 


97 


The Testing of Rubber Goods 

Rubber. —The determination of the amount of rubber present 
in a vulcanized compound is an important though difficult matter. 
For a long time this was determined by igniting a weighed sample 
and determining the mineral fillers or ash. The rubber was cal¬ 
culated by the difference between ioo per cent and the sum of the 
ash (sulphur free), total sulphur, and corrected acetone extract. 
This procedure is still extensively used, and, although it can not 
be depended upon always to give accurate results, it is probably 
as good as any method yet devised. 

The problem is to-day being attacked from several points. 
Some chemists are endeavoring to find a suitable solvent which 
will remove the rubber and permit the weighing of the mineral 
residue. Turpentine, terebene, cymene, anisole, phenetole, cresol, 
aniline, and many others have been used with more or less success. 
Those compounds which contain ingredients such as glue, carbon, 
cellulose, and antimony sulphide, which will burn or volatilize 
during ignition, must be analyzed by use of a suitable solvent. 
This Bureau has had good success with the methods described 
further on. Other chemists are working along the line of the 
direct determination of the rubber present by means of the vari¬ 
ous addition products, such as tetrabromide, nitrosite, etc., but 
the methods are not yet satisfactory. 

Specific Gravity. —The specific gravities of the various con¬ 
stituents of vulcanized rubber differ greatly. (See Appendix.) 
The percentages found by analysis are, however, always expressed 
by weight. It is apparent, therefore, that with equal percent¬ 
ages, by weight, a compound of specific gravity of, say, 1.5 will 
have less rubber per unit volume than one of a higher specific 
gravity. The present tendency is to state the minimum percent¬ 
age of rubber by volume. 

Waxy Hydrocarbons. —The efficiency of rubber as an insu¬ 
lator is very much lessened by the absorption of moisture. To 
prevent this, small amounts of paraffin or ceresin are added. 
Their presence is usually permitted in specifications, the limit 
being generally placed at 4 per cent, although the tests made in 
this Bureau indicate that the maximum amount is seldom used. 
The amount of such waxy hydrocarbons is determined not only in 
order to learn if the specifications have been complied with, but 
also to obtain the percentage of acetone extract from the rubber 
itself. 

56597°—21-7 



98 


Circular of the Bureau of Standards 

The chloroform extraction removes a large portion of the min¬ 
eral rubbers, which are only partially soluble in acetone. Only 
a small amount of properly vulcanized rubber of good quality is 
dissolved by the chloroform. 

Alcoholic Soda Extract. —Some of the rubber substitutes 
are prepared by the action of sulphur or sulphur chloride on vege¬ 
table oils. The purpose of the alcoholic-potash extraction is to 
detect the presence of such substitutes and to give some idea of 
their amount. Hevea rubber contains only a small percentage of 
material extracted by this solvent. 

(b) SAMPLING 

1. (i) Preparation. —Before preparing a sample for analysis 
the analyst shall, by inspection, assure himself that it has not 
been contaminated in any way before reaching him. 

2. Not less than 25 grams of the sample shall be prepared by 
taking pieces from various parts of the original material. This 
shall be separated as far as possible from foreign matter—that 
is, different grades of rubber, backing, friction, cloth, wire, and 
the like—by stripping or buffing. Pieces shall be laid aside for 
the specific-gravity determination before grinding the sample. 

3. Samples of soft rubber shall be prepared by breaking down 
on the experimental mill to the required fineness. Care must be 
taken not to overheat the sample. With compounds of high 
rubber content such as inner tubes, and floating stocks, the re¬ 
quired fineness is usually attained when the sample has been 
sheeted out very thin. If no mill is available, the rubber is to be 
cut very fine with scissors, or if heating can be avoided, it may 
be run through a meat chopper. 

4. Samples of hard rubber shall be prepared for analysis by 
rasping or buffing. 

5. Raw, reclaimed, or unvulcanized rubber shall be sheeted out 
very thin on the experimental mill and shall be rolled in holland 
or other cotton cloth to prevent the sample from sticking. It 
may be also cut with scissors. 

6. Samples of rubberized cloth shall be taken from various 
parts of the original material and prepared by cutting into pieces 
1 f/ 2 mm square with the scissors and then well mixed. 

7. Samples prepared for the determination of rubber by the 
nitrosite method shall be ground to pass a 20-mesh screen or cut 
with the scissors to the smallest size practicable. 


The Testing of Rubber Goods 


99 


8. (2) Classification. —Samples of rubber goods may be 
classified roughly as follows: Those for which the rubber content 
may be arrived at by going through the regular procedure of anal¬ 
ysis; that is, specific gravity, acetone extract, chloroform extract, 
alcoholic-soda extract, free sulphur, total sulphur, ash, sulphur 
in ash, and free carbon. 

9. Those for which the regular procedure must be augmented 
by the addition of special analyses to determine those substances 
which decompose when the rubber is ashed. The members of 
this class may contain antimony sulphide, glue, cloth, cork, etc. 

10. Those which must be analyzed according to other required 
specifications, as some insulation, code wires, etc. 

11. Those which contain a solvent or thinner, as pastes, put¬ 
ties, cements. The solvent or thinner is evaporated, and the 
dried residue is then analyzed as an unvulcanized sample; it 
usually comes under the class described in paragraph 8. The 
quantity and composition of the solvent or thinner should be 
determined. 

12. Those which require the Joint Rubber Insulation Committee 
method. This class includes compounds which are high in car¬ 
bonates and sulphides as well as those for which the above method 
is specifically requested. 

(c) REAGENTS 

13. The acetone used for extractions shall be chemically pure 
and shall be freshly redistilled over anhydrous sodium carbonate. 
Acetone to be used for the determination of ash, free carbon, and 
nitrogen may be recovered acetone which has been redistilled as 
above. 

14. The alcoholic-potash or alcoholic-soda solution shall be of 
normal strength and shall be freshly made by dissolving the 
required amount of alkali in the smallest possible quantity of 
distilled water and adding this to specially purified alcohol. The 
alcohol for this purpose shall be prepared by allowing an approx¬ 
imately twice normal alcoholic solution of sodium hydroxide to 
remain in a warm place, preferably at 50 to 6o° C, for several 
weeks and then redistilling. 

15. The nitric acid-bromine reagent shall be prepared by adding 
a considerable excess of bromine to concentrated nitric acid, 
shaking thoroughly, and allowing it to stand some hours before 
using. 


ioo Circular of the Bureau of Standards 

16. The fusion mixture for sulphur determinations shall be 
made bv mixing equal quantities of sodium carbonate and pow¬ 
dered potassium nitrate. According to the alternative method 
(par. 27) only sodium carbonate is used. 

17. Barium-chloride solution shall be made by dissolving 100 
grams of crystallized barium chloride in 1 liter of distilled water 
and adding 2 or 3 drops of concentrated hydrochloric acid. If 
there is any insoluble matter or cloudiness, the solution shall be 
heated on the steam bath overnight and filtered. Care should be 
taken not to add more acid than the amount specified. 

18. If sulphur is to be determined, the carbon bisulphide must 
be redistilled through a fractionating column filled with small 
pieces of bright metallic copper. 

19. All other reagents shall be chemically pure and should be 
tested before use. 

(d) ANALYSIS 

20. (1) Qualitative. —In order to properly classify the sam¬ 
ple, a rough qualitative analysis is necessary. The color and 
nature of the sample should be noted when beginning an analy¬ 
sis, so as to get an indication of the presence of carbon, antimony 
sulphide, vermilion, cloth, and cork. In order to determine 
whether glue is present, the qualitative test (par. 45) must be 
carried out. Also by close observation during the course of an 
analysis considerable qualitative information as to the constitu¬ 
ents may be gained. The appearance of the extracts, the color 
of the fusions, the color of the ash, both hot and cold, should 
always be noted. If the complete mineral analysis is desired, 
time is saved by first making a qualitative analysis. There are 
a number of standard books which may be referred to for this 
purpose. 

21. (2) Quantitative. — (a) General Procedure .—Specific Grav¬ 
ity.—If plenty of the material is available a piece weighing about 
25 grams is cut off, suspended from the balance by a very fine 
silk thread, and weighed to the third decimal place. The sample 
is then immersed for two to three minutes in boiling water, re¬ 
moved, and cooled. It is suspended from the balance by the silk 
thread used above and weighed while completely immersed in 
distilled water at 15 0 C. The specific gravity is found by divid¬ 
ing the weight of the sample in air by the difference between its 
weight in air and water. For very porous material it is well to 
have the water in a flask and after short boiling to connect the 


The Testing of Rubber Goods ioi 

flask to the vacuum line. This will cause the water to boil at a 
gradually decreasing temperature and will assist greatly in remov¬ 
ing air bubbles from the rubber. If only a small amount of the 
rubber is available, the determination is made by means of the 
pycnometer bottle. About 5 grams of the sample is cut into 
small strips and weighed. It is treated as before to remove air 
bubbles. A pycnometer bottle is filled with water at 15 0 C and 
weighed. The sample is placed in the bottle, which is again filled 



Fig. 47 .—Acetone and chloroform extraction apparatus 

For the extraction of the rubber resins, free sulphur, any mineral oils or waxes, and part of any 

bituminous substances or vulcanized oils 


with water and weighed. The specific gravity is calculated as 
follow's: 

A = weight of pycnometer when filled with water; 

B = weight of pycnometer filled with sample and water; 

C = weight of sample. 

Sp ' gr ‘ = C-{B-A) 

22. Acetone Extract.—Place 2 grams of rubber into a thimble 
made by folding a 9-cm filter paper, so that it will fit into the 
extraction cup, which is suspended in a weighed assay flask. (See 
Fig. 47.) Extract the sample continuously for eight hours unless 
the solution in the thimble is still colored at the end of that time, 










102 Circular of the Bureau of Standards 

when the extraction shall proceed for a further period of four 
hours or longer. Carefully note all characteristics of the acetone 
extract, both when hot and cold. Distil off most of the acetone 
in a recovery still and drive off the remaining solvent on the 
steam bath at as low a temperature as possible by means of a 
gentle current of air. Care must be taken to avoid allowing the 
flasks to stand on the steam bath after the solvent has been 
removed and while the air is still passing through, because appre¬ 
ciable quantities of free sulphur may be lost by so doing. Dry 
the assay flask and contents in an air bath for one-half hour at 
90 to 95 0 C, cool, and weigh. Call the residue “acetone extract, 
uncorrected.” To calculate, divide the residue in grams by the 
weight of the original sample. 

23. Chloroform Extract.—The rubber sample, without remov¬ 
ing the acetone from it (see par. 22), is suspended in a second 
weighed assay flask (see Fig. 47) and extracted for one hour with 
chloroform. If the solution in the extraction bucket is colored, 
the extraction is continued until it becomes colorless. Care should 
be taken that any small particles of rubber, which are often car¬ 
ried down into the extract, are filtered off. Evaporate off the 
solvent (see par. 22), dry for one-half hour at 90 to 95 0 C, cool, 
and weigh. The color of the chloroform solution should be re¬ 
corded in the laboratory notebook. Reserve the rubber for ex¬ 
traction with alcoholic soda. (See par. 24.) 

24. Alcoholic-Soda Extract.—Dry the rubber at about 50 to 
6o° C, transfer to a 200 cc Erlenmeyer flask, add 50 cc of alcoholic- 
soda solution, and heat under a reflux condenser for four hours. 
Filter through a pleated filter into a 250 cc beaker, wash first with 
50 cc of 95 per cent alcohol, then with 50 cc of boiling water, and 
evaporate the filtrate to dryness. Use about 75 cc of distilled 
water to transfer the residue to a separatory funnel. Add a few 
drops of methyl orange, and acidify the solution with 10 per cent 
hydrochloric acid. Extract with four portions of ether, 25 cc 
each, unless the fourth portion should be colored, when the extrac¬ 
tion must be continued until no further quantity can be removed. 
Unite the ether fractions, wash thoroughly with distilled water, 
and evaporate the ether in an Erlenmeyer flask. Dry at 90 to 
95 0 C, cool, and weigh. 

25. Free Sulphur.—Add to the flask containing the acetone 
extract, uncorrected (see par. 22), 50 to 60 cc of distilled water 
and 2 or 3 cc of bromine. (If the acetone extract indicated a large 


The Testing of Rubber Goods 


103 


amount of free sulphur, the amount of bromine used may be in¬ 
creased.) Allow the flask to stand one-half hour on the side of the 
steam bath, then heat cautiously over the direct steam bath until 
the solution is practically colorless, and filter through a pleated 
filter into a 250 cc beaker. Cover the beaker with a watch glass, 
heat on the steam bath, add 10 cc of hot 10 per cent barium- 
chloride solution, and allow the precipitate to stand overnight. 
The next day filter the precipitate on a 9 cm filter paper, and wash 
until free from chlorides. Ignite in a small porcelain crucible 
over a Bunsen flame or in a muffle furnace, but do not allow the 
paper to inflame; cool, and weigh. Calculate the barium sulphate 
to sulphur by means of the factor 0.1373, and calculate the per¬ 
centage of free sulphur. 

26. Total Sulphur. 5 —Place 0.5 gram of rubber in a porcelain 
crucible of about 75 cc capacity. Add 20 cc of the nitric acid- 
bromine mixture (see par. 15), cover the crucible with a watch 
glass, and let it stand for one hour in the cold. Heat for one hour 
on the steam bath, remove the cover, rinse it with a little distilled 
water, and evaporate to dryness. Add 5 grams (weigh to about 
0.5 gram) of fusion mixture (see par. 16) and 3 to 4 cc of distilled 
water, and stir at once. Digest for a few minutes, spread the mix¬ 
ture halfway up the side of the crucible to facilitate drying, and 
dry on a steam bath. Fuse the mixture by heating either over a 
sulphur-free gasoline flame, or in a muffle furnace, the temperature 
of which should be regulated by a rheostat and determined by a 
pyrometer. 

If the flame is used, place the crucible in an inclined position 
on a wire triangle and start the ignition over a low flame. The 
tendency for the rubber to bum too briskly is controlled by judi¬ 
cious use of the stirring rod, which scrapes the burning portion 
away from the rest. When part of the mass is burned white, 
a fresh portion is worked into it, and so on until all of the organic 
matter is destroyed. It is necessary to hold the edge of the crucible 
with tongs. Toward the last half of the operation the flame should 
be increased somewhat, but it is never necessary to heat the cruci¬ 
ble to redness. With care a crucible can be used for at least 10 or 
12 fusions. 

When a furnace is used the crucible should be covered with a 
watch glass to prevent loss by spattering. The crucible is set on 


6 Waters and Tuttle. B. S. Sci. Paper. No. 174- See also Tuttle and Isaacs. B. S. Tech. Paper, No. 45. 
For the most accurate work occluded salts can be corrected for by adapting the suggestion of Johnson and 
Adams. J. Am. Chem. Soc., 33 , pp. 829-845; i 9 ii- See also Waters, B. S. Tech. Paper. No. 177. 




104 


Circular of the Bureau of Standards 


a low tripod made from an iron or copper wire triangle with the 
ends bent down. Several of these crucibles can be placed in an 
iron tray which will protect the lining of the muffle from any 
fusion mixture if a crucible should break. The rate of heating 
of the muffle should be as shown below. For convenience the rate 
of heating on igniting to ash is also given. 


TABLE 10.—Rate of Heating of the Electric Furnace for the Sulphur and Ash 

Determinations 


Time in minutes 

Sulphur 

determi¬ 

nation 

tempera¬ 

ture 

Ash de¬ 
termina¬ 
tion tem¬ 
perature 

Time in minutes 

Sulphur 

determi¬ 

nation 

tempera¬ 

ture 

Ash de¬ 
termina¬ 
tion tem¬ 
perature 

5. 

°C 

130 

320 

345 

350 

370 

390 

410 

430 

°C 

130 

320 

345 

350 

370 

390 

410 

430 

70. 

°C 

450 

470 

490 

500 

°C 

450 

470 

490 

525 

595 

640 

690 

10. 

80. 

IS. 

90. 

20. 

30. 

95. 

40. 

100 . 

110. 


50. 

115. 


60. 





After a fusion by either method allow the crucible to cool, 
place it in a 400-cc beaker, add sufficient distilled water to cover 
the crucible (about 125 cc is required), and digest on the steam 
bath for two hours with occasional stirring. If the filtration can 
not be made in the same day do not add the water and allow the 
mixture to stand overnight. Filter the solution into a covered 
400-cc beaker containing 8 cc of concentrated hydrochloric acid, 
and wash the residue thoroughly with hot water. Now complete 
the acidification of the filtrate and washings and add 2 cc of con¬ 
centrated hydrochloric acid in excess. Cover the beaker and 
heat the solution on the steam bath. The total volume of the 
solution should not exceed 300 cc. The solution must be acid to 
congo paper in order to insure the complete destruction of the 
carbonates. Add 10 cc of hot 10 per cent barium-chloride solu¬ 
tion and allow to stand overnight. Filter off the barium sulphate 
as before. (See par. 25.) Calculate to the percentage of sulphur 
present. 

27. Alternative Method 6 .—Place 0.5 gram of rubber in a 75 cc 
porcelain crucible. Add 10 cc of nitric acid-bromine mixture 
(see par. 15), cover the crucible with a watch glass, and let it 
stand one hour in the cold. Heat on steam bath for one hour, 


6 Waters, J. Ind. Eng. Chem., 12 , pp. 482-485; 1920. See also B. S. Tech. Paper, No. 177. See also 
Levin and Collier. Rubber Age and Tire News, 9 , 2, pp. 47-48; 1921. 
































The Testing of Rubber Goods 105 

uncover and evaporate to dryness. Add 3 cc of nitric acid, cover, 
warm a short time on bath, then let it cool. Carefully add in small 
portions, by means of a small glass spatula, 5 grams of sodium 
carbonate (weighed to 0.5 gram). The watch glass is to be raised 
only high enough to permit of the introduction of the spatula. 
The carbonate is allowed to slide down the side of the'crucible and 
is not dropped directly into the acid. Rinse the watch glass with 
2 or 3 cc of hot distilled water and stir the mixture thoroughly with 
a glass rod. Digest for a few minutes, spread the mixture halfway 
up the side of the crucible to facilitate drying, and dry on the steam 
bath. Fuse the mixture. (See par. 26.) Allow the crucible to 
cool, place it in a 400-cc beaker, add sufficient distilled water to 
cover the crucible (about 125 cc is required), and digest on the 
steam bath for two hours, with occasional stirring of the solution. 
If the filtration can not be made the same day do not add the water, 
but allow the mixture to remain overnight. Filter into a 400-cc 
beaker, containing 5 cc of concentrated hydrochloric acid, and 
wash the residue thoroughly with hot water. Add sufficient 
concentrated hydrochloric acid to neutralize the excess carbon¬ 
ates, using congo-red paper as an indicator, and add 2 cc in excess. 
Dilute the solution to 300 cc. A strip of gummed paper may be 
placed on the beaker to show the 300-cc level. Heat on the steam 
bath, add 10 cc of hot 10 per cent barium-chloride solution, and 
allow to stand overnight. Filter off the precipitate, wash, and 
ignite in the usual way. 

28. Ash.—Wrap a 1 gram sample in an 11 cm filter paper, 
extract with acetone for four hours (recovered acetone can be 
used), and transfer to a weighed, medium-sized porcelain crucible. 
Distil off the rubber over a very small flame, not allowing it to 
catch fire, and ignite until burnt clean; cool, and weigh. 

The sample may be ashed in a muffle furnace if preferable. 
The rate of heating should be as given in Table 10. (See par. 26.) 

29. Sulphur in Ash.—Add 3 cc of nitric acid-bromine mixture 
(see par. 15) to the ash (see par. 28), heat on the steam bath, and 
after one hour remove the crucible and allow it to cool. Carefully 
add 5 grams of sodium carbonate as described in paragraph 27, and 
finish as under paragraph 27. Save the insoluble residue on filter¬ 
ing the solution of the fusion mixture in water for testing according 

to paragraph 43. 

30. Free Carbon 7 .—Extract a 0.5-gram sample for eight hours 
with a mixture of 68 per cent chloroform and 32 per cent acetone, 


7 Smith and Epstein. B. S. Tech. Paper, No. 136. 






106 Circular of the Bureau of Standards 

by volume. Transfer the sample to a 250 cc beaker and heat on 
the steam bath until it no longer smells of chloroform. Add a 
few cubic centimeters of hot concentrated nitric acid and allow to 
stand in the cold for about 10 minutes. Add 50 cc more of hot 
concentrated nitric acid, taking care to wash down the sides of 
the beaker, and heat on the steam bath for at least one hour. 
At the end of this time there should be no more bubbles or foam 
on the surface. Pour the liquid while hot into a Gooch crucible 
containing a thick pad of asbestos, taking care to keep the insol¬ 
uble material completely in the beaker. Filter by slowly applying 
gentle suction and wash well by decantation with hot concen¬ 
trated nitric acid. Empty the filter flask. Wash with acetone 
and a mixture of equal parts of acetone and chloroform until the 
filtrate is colorless. The insoluble material, which has been care¬ 
fully retained ip the beaker, is digested for 30 minutes on the 
steam bath with 35 cc of a 25 per cent solution of sodium hy¬ 
droxide. Dilute to 60 cc with hot distilled water and heat on 
the steam bath. Filter the alkaline solution and wash well with a 
hot 15 per cent solution of sodium hydroxide. Test for the pres¬ 
ence of lead by running some warm ammonium-acetate solution, 
containing an excess of ammonium hydroxide, through the pad 
into a solution of sodium chromate. If a yellow precipitate ap¬ 
pears, the pad must be washed with the ammonium-acetate solu¬ 
tion until the washings no longer precipitate the sodium-chromate 
solution. Next wash the residue a few times with hot concentra¬ 
ted hydrochloric acid, and finally vith a warm, 5 per cent hydro¬ 
chloric-acid solution. Remove the crucible from the funnel, 
taking care that the outside is perfectly clean, dry it in an 
air bath for one and one-half hours at 150° C, cool, weigh, 
burn off the carbon at a dull red heat, and re weigh. The differ¬ 
ence in weight represents approximately 105 per cent of the car¬ 
bon originally present in the form of lampblack or gas black. 

31. Rubber Hydrocarbons.—Calculated by difference: Sub¬ 
tract from 100 per cent the sum of the following: Chloroform ex¬ 
tract; alcoholic soda or potash extract; acetone extract, corrected; 
ash, sulphur free; total sulphur; and free carbon. Call the figure 
thus obtained “rubber hydrocarbon by difference.” The method 
of obtaining the corrected figures for ash and acetone extract used 
above is as follows: (1) Subtract the “free sulphur” from the 
“acetone extract, uncorrected” and report the difference as 
“acetone extract, corrected.” (2) Subtract the “sulphur in ash” 
from the “ash” and report the difference as “ash, sulphur free.” 


The Testing of Rubber Goods 


107 


(3) Subtract from the total sulphur the percentage of sulphur 
present as barytes, if the latter determination has been made (see 
par. 43), and report the difference as “total sulphur corrected.” 
Then add the sulphur so deducted to the ash, in this case reporting 
the latter simply as “ash corrected.” In other words, only the 
sulphur other than barytes will be deducted from the ash when the 
total sulphur is corrected for barytes. 

32. Rubber as Compounded.—Rubber hydrocarbon plus 5 per 
cent of its weight is taken as “rubber as compounded,” except 
when the sum of the acetone extract, corrected, chloroform extract, 
and alcoholic-soda extract is less than the figure represented by the 
arbitrary 5 per cent, as in many high-grade compounds. Rubber 
as compounded is then: Rubber hydrocarbon plus the sum of the 
acetone extract, corrected, the chloroform, and the alcoholic-soda 
extract. 

33. Rubber by Volume.—Rubber as compounded (determined 
as in the preceding paragraph) is multiplied by the specific gravity 
of the compound and divided by 0.94 (taken as the average specific 
gravity of crude rubber). The result expresses the percentage of 
rubber by volume. 

34. Ratio of Acetone Extract to Rubber.—This is calculated by 
dividing the percentage of the acetone extract, corrected, by the 
percentage of rubber as compounded, and the quotient, expressed 
in percentage, gives approximately the corrected acetone extract 
of the rubber used in the compound. 

35. (6) Procedure for the Analysis of 30 or 40 per cent Para Insu¬ 
lation. —The determinations made on high-grade insulation com¬ 
pounds are acetone extract, unsaponifiable matter, waxy hydro¬ 
carbons, free sulphur, ash, and total sulphur. 

36. Acetone Extract.—Determine as under paragraph 22. 

37. Unsaponifiable Matter.—Add to the acetone extract (see 
par. 36) 50 cc of normal alcoholic potash (see par. 14), heat on the 
steam bath under a reflux condenser for two hours, remove the 
condenser, and evaporate to dryness. Transfer to a separatory 
funnel, using about 100 cc of water, add 25 cc of ether, and shake. 
Allow the two layers to separate thoroughly, then draw off the 
water layer. Continue the extraction of the water layer with fresh 
portions of ether until no more unsaponifiable matter is removed, 
unite the ethereal layers, and wash with distilled water, adding the 
first wash water to the extracted aqueous layer. This aqueous 
solution is reserved for the free-sulphur determination. (See par. 
39.) Transfer the ether to a tared Erlenmeyer flask (it will be 


108 Circular of the Bureau of Standards 

found convenient to use the flask in which the acetone extract was 
weighed), distil off the ether, dry one-half hour at 90 to 95 0 C, 
cool, and weigh. 

38. Waxy Hydrocarbons.—To the unsaponifiable matter (see 
par. 37) add 50 cc of absolute alcohol and heat on the steam bath 
for one-half hour. Place the flask in a mixture of ice and salt and 
let stand for one hour. Filter off the separated waxy hydrocar¬ 
bons, using filter paper and applying a gentle suction. Wash 
with alcohol (95 per cent will do) which has been cooled in an 
ice-salt mixture. The funnel should be surrounded by a freezing 
mixture in order that the temperature may not rise during filtra¬ 
tion. Dissolve the precipitate from the filter paper with hot 
chloroform and catch the solution in a weighed 100 to 150 cc 
beaker. Wash the flask with hot chloroform, which is added to 
the same beaker, in order to include any waxy matter adhering 
to the walls of the flask. Evaporate off the solvent, dry the resi¬ 
due at 90 to 95 0 C, cool, and weigh. 

39. Free Sulphur.—Transfer the water layer (see par. 37) to a 
250-cc beaker and heat on the steam bath until the ether has been 
removed. Add 25 cc of bromine water, heat one hour, add 5 cc 
of concentrated hydrochloric acid, and heat until the excess of 
bromine has been driven off. (Test for acidity with congo paper; 
the amount of acid specified is sufficient if instructions are followed 
exactly; a large excess of acid is to be avoided.) Filter into a 250- 
cc beaker, add 10 cc of hot 10 per cent barium-chloride solution, 
and finish the determination as under paragraph 25. 

40. Ash.—Proceed as under paragraphs 28 and 29. 

41. Total Sulphur.—Proceed as under paragraph 26. 

42. Calculations.—(1) Subtract the sum of the free sulphur 

and waxy hydrocarbons from the acetone extract, uncorrected, and 
report the difference as “acetone extract, corrected.” (2) Sub¬ 
tract from 100 per cent the sum of the acetone extract, corrected, 
waxy hydrocarbons, ash—sulphur free, and total sulphur—and 
report the result as “rubber by difference.” (3) Calculate the 
“rubber as compounded” as under paragraph 32. (4) Calculate 

the “ ratio of the acetone extract to rubber ” as under paragraph 34. 

43. (c) Special Determinations. —Barytes.—The barytes is cal¬ 
culated from the barium in the ash; this is determined as follows: 
Filter off the insoluble matter after the fusion and extraction in 
paragraph 29, wash back into the original beaker with hot water. 
Dissolve the residue in the beaker and any traces on the filter 
paper with hydrochloric acid, and heat the solution on the steam 


The Testing of Rubber Goods 


109 


bath. Filter through the same filter as before and wash thoroughly 
with hot water. Nearly neutralize the solution with ammonium 
hydroxide, leaving it slightly acid. Saturate the solution with 
hydrogen sulphide in the cold, and when the lead sulphide has 
settled filter into a 400-cc beaker and wash thoroughly. The 
total volume should not be over 200 cc. Cover the beaker con¬ 
taining the filtrate, heat to boiling, and add 10 cc of hot 10 per 
cent sulphuric acid. Allow the precipitate to stand overnight. 
Filter off the barium sulphate as directed in paragraph 25. Calcu¬ 
late the percentage of barytes. Then calculate the percentage of 
sulphur in the barytes by the factor 0.1373. 

44. Barium Carbonate. 8 —Barytes was determined by the calcu¬ 
lation to barytes of all barium found in the sample. Obviously 
if barium carbonate is present, it must be determined in order 
that an undue correction will not be made. The determination 
is as follows: A i-gram sample, in a porcelain boat, is placed in a 
combustion tube through which passes a current of carbon dioxide. 
The sample is ashed in the tube. After ignition and cooling in 
the atmosphere of carbon dioxide, the boat is removed, the residue 
finely ground in an agate mortar, transferred to a 250-cc beaker, 
and treated with 5 to 10 grams of ammonium carbonate, 15 to 20 
cc of strong ammonia water, and about 50 cc of distilled water. 
The mixture is boiled for 20 minutes, filtered, and the precipitate 
thoroughly washed to remove all soluble sulphates. The residue 
on the filter paper is washed back into the original beaker and about 
10 cc of glacial acetic acid with sufficient water to make the total 
volume about 100 cc is added. This is heated to boiling and fil¬ 
tered through the same paper as before. Hydrogen sulphide is 
passed into the filtrate to precipitate the lead and the solution is 
subsequently treated as in the determination of barytes. The 
final weight of barium sulphate obtained is calculated to barium 
carbonate. 

45. Glue Detection.—About 0.5 gram of the rubber sample to 
be tested for the presence of glue is digested in 25 cc of freshly 
distilled cresol (b. p., 195 0 C) in a tall beaker for about 16 hours at 
120 0 C. This is most conveniently done in a properly regulated 
Freas oven overnight. The cresol solution is allowed to cool, and 
250 cc of petroleum ether is added slowly, with constant agitation. 
When this solution has settled and the supernatant liquid is clear, 
it is filtered through a Gooch crucible, using gentle suction. The 


8 Tuttle, B. S. Tech. Paper No. 64. 





no Circular of the Bureau of Standards 

beaker and contents and the crucible are washed thoroughly with 
petroleum ether, then with hot benzene. The pad is removed 
from the crucible, placed inside of the beaker, and digested for 
several minutes with boiling water. The solution is filtered 
through a pleated filter paper, and the filtrate is to be used for the 
glue test. The volume of this solution should not exceed 150 cc. 
After it has cooled it is poured very slowly into a concentrated 
solution of tannic acid. In the presence of glue a permanent 
cloudiness will appear at first, and finally a precipitate as more of 
the glue solution is added. Large percentages of glue give heavy 
curdy precipitates, while small quantities give a decided cloudi¬ 
ness. In the absence of glue no permanent precipitate or cloudi¬ 
ness will appear immediately in the tannic-acid solution as the 
liquid is added to it. 

46. Glue Determination.—Extract a 2-gram sample with a 
mixture of 68 per cent of chloroform and 32 per cent of acetone 
for six hours. Remove the solvents from the sample and transfer 
the latter to a 750-cc Kjeldahl flask. Add 25 to 30 cc of concen¬ 
trated sulphuric acid, 10 to 12 grams of sodium sulphate, and 
about 1 gram of copper sulphate. Place the flask on the Kjeldahl 
digesting apparatus and heat gently until the first vigorous froth¬ 
ing ceases, then raise the heat gradually until the liquid boils. 
Continue the boiling until the solution becomes clear. Allow the 
flask to cool to 40 to 6o° C (if allowed to become thoroughly cold, 
the solution solidifies), dilute carefully with 150 cc of water, and 
allow to cool. Add 100 cc of 50 per cent sodium-hydroxide solu¬ 
tion, pouring it carefully down the side of the flask, so that it does 
not mix immediately with the acid solution. Add about 1 gram 
of granulated zinc to prevent bumping and a piece of paraffin the 
size of a pea to diminish frothing. Connect the flask quickly with 
a condenser the delivery tube of which dips into a 500-cc Erlen- 
meyer flask containing 50 cc of N/10 sulphuric acid diluted to 
about 100 cc. Shake up the Kjeldahl flask and heat gently at 
first, increase the flame as the danger of foaming over diminishes, 
and finally boil briskly until about one-half of the liquid has passed 
over into the receiver. Add methyl-red solution and titrate the 
excess acid by means of tenth-normal sodium-hydroxide sblution. 

Calculation: 

100 (cc H 2 S 0 4 Xnormality — cc NaOHXnormality) (0.014) (5.56) 

weighTofsainple--=percentage glue. 



The Testing of Rubber Goods 


111 


47. Rubber by Nitrosite Combustion. 9 —Preliminary extrac¬ 
tions of a 2-gram sample are made according to the usual methods 
to determine the amount of acetone, chloroform, and alcoholic- 
potash extracts. These extractions will prove the presence or 
absence of mineral rubbers and oil substitutes. In the absence 
of both, proceed as below. If mineral rubbers are present, make a 
chloroform extraction; if oil substitutes, an alcoholic-potash 
extraction; and if both, make both extractions. In every case 
make these extractions after the acetone extraction and before 
the sample is allowed to swell in chloroform. When an alcoholic- 
potash extraction is made, wash the sample thoroughly with 
5 per cent hydrochloric acid, hot water, and alcohol. 

48. Take 0.500 to 1 gram of the finely ground sample (call 
this weight W) and extract with acetone for eight hours. Dry 
the residue in hydrogen (or other inert gas) for two hours at 100 0 C. 
Place the sample in 50 to 75 cc of chloroform and allow it to 
swell. Pass into this until the green color which is formed per¬ 
sists for 30 minutes the gases formed by heating arsenic trioxide 
and nitric acid of specific gravity 1.30. To avoid contamination 
it is important that no rubber connections be used. Immerse 
the flask containing the rubber in cold water during the nitra¬ 
tion. Allow the solution to stand overnight; the next day filter 
off the nitrosite through a Gooch crucible and wash with small 
quantities of chloroform. Remove the acid gases and chloro¬ 
form from the flask by means of a gentle current of air. Evapo¬ 
rate the filtrate to dryness. Dissolve the nitrosite remaining in 
the flask in the Gooch crucible and in the residue from the filtrate 
in acetone and filter the solution through asbestos into a weight 
burette. The total volume should be about 100 cc. Allow this 
solution to stand a short time so as to permit any sediment which 
may form to settle out in the bottom of the weight burette. Weigh 
the burette before and after filling, calling the difference N. Draw 
off about 25 cc into a small Erlenmeyer flask, reweigh the burette, 
and call the difference 0 . Evaporate the portion drawn off to a 
small volume, transfer to a porcelain boat (about 14 cm long 
and 1 cm wide) which has been filled with alundum, and wash 
the flask with acetone. (It is best to make this transfer in small 
portions, drying the boat and contents for a few minutes between 
each addition.) After the final washing and drying add 1 or 2 
cc of 1 per cent solution of ammonia in distilled water, and dry in an 


1 Wesson, B. S. Tech. Paper, No. 35- See also Tuttle and Yurow, B. S. Tech. Paper, No. 145• 





112 Circular of the Bureau of Standards 

inert gas for one hour at 90° C. Repeat with a second portion of 
ammonia and dry as before. By this means all of the organic 
solvent will be removed. 

49. Place the boat in the furnace and proceed with the combus¬ 
tion as usual. The furnace should be carefully tested before using. 
Pass the products of combustion through U-tubes or other satis¬ 
factory absorption tubes placed in the following order: a, b, c, 
potassium bichromate-sulphuric acid; d, powdered zinc, 20-mesh; 
e, f, soda lime and calcium chloride; g, potassium bichromate- 
sulphuric acid; h, dilute palladium-chloride solution. (Very little 
palladium chloride is needed. Use about 1 drop of a 10 per cent 
solution in 10 cc of distilled water.) Weigh e, f, and g before and 
after each combustion; refill c and g frequently from the same so¬ 
lution, so that the gases which enter e and those that leave g will 
have the same moisture content. The palladium chloride serves to 
detect the presence of carbon monoxide or other reducing gases; 
if there is any blackening, it shows incomplete oxidation. In 
this event, discard the results and repeat the determination. 

50. The carbon dioxide will equal the algebraic sum of the 
differences in tubes e, /, and g. Call this p. The factor for cal¬ 
culating from carbon dioxide to rubber hydrocarbon is 0.309. 
The formula is therefore as follows: P X 0.309 X N/Ox 100/W = 
percentage of rubber hydrocarbon. Correct this figure for what¬ 
ever extractions were made previous to nitration. 

51. Total Fillers, Uncorrected.—Extract a 0.5-gram sample 
with acetone for eight hours. If the extract indicates that bi¬ 
tuminous substances are present, the sample should be further 
extracted with chloroform until the chloroform in the extrac¬ 
tion bucket is colorless. Evaporate off the excess solvent from 
the rubber samples by heating on a steam bath for a short time, 
and finally remove in a vacuum desiccator the last traces of 
the solvent. Transfer the sample to a tared 150-cc lipped assay 
flask, add 25 cc of cymene, and heat on an electric hot plate or 
in an oven at 130 to 140° C until the rubber has completely dis¬ 
solved, as shown by the fillers settling out and the supernatant 
liquor becoming almost clear. It has been found that the time 
required for complete solution varies with different cures and differ¬ 
ent compounds. As a rule, six hours is sufficient. Remove the 
flask, allow the solution to cool, dilute with 100 cc of petroleum 
ether, and allow the solution to stand overnight. Prepare a 
Gooch crucible with a tight pad made of asbestos which has been 


The Testing of Rubber Goods 113 

I 

treated previously with 10 per cent caustic-soda solution, con¬ 
centrated hydrochloric acid, and finally water. Carefully wipe 
off the crucible, dry, ignite, and weigh. 

For the best results it is necessary to filter slowly by means of 
gentle suction. Collect the filtrate in a 500-cc Erlenmeyer flask 
under a bell jar and set it aside for the determination of the sulphur 
of vulcanization. Keep as much as possible of the fillers in the 
assay flask. Wash with about 50 cc of petroleum ether, using 
about 10 cc at a time. Then wash with hot benzene, using 5 cc 
at a time, and finally with hot acetone. In each case the washing 
should be continued until the solvent comes through colorless. 
Care must be taken to see that the bottom of the crucible is not 
in contact with the rubber holder, because of the solvent action 
of the benzene and acetone. Carefully wipe off the sides of the 
flask and crucible with a cloth moistened with benzene, dry for 
one hour at ioo° C, cool, and weigh. Subtract the weight of the 
empty crucible and flask and record the difference as total fillers, 
uncorrected. 

52. Total Fillers, Corrected.—Digest the contents of'the flask 
and crucible from the determination of total fillers, uncorrected 
(see par. 51), with a 25 per cent solution of sodium hydroxide for 
30 minutes on the steam bath. Dilute to 60 cc with hot distilled 
water and heat on the steam bath. Filter through a Gooch 
crucible, wash twice with hot water and then very thoroughly 
with hot concentrated hydrochloric acid. If silicates are not 
present, the digestion with sodium hydroxide is unnecessary. 
Care must be taken in adding the first portion of acid, since in the 
presence of carbonates the effervescence which ensues may cause 
some of the fillers to be lost. It is best to add the acid a few 
drops at a time until the effervescence is over and then proceed 
with the washings in the usual way. Wash at least 10 times with 
boiling distilled water and transfer the contents of the flask to the 
crucible with the aid of a “policeman.” Dry for one hour at 
105 0 C, cool, and weigh; ignite and reweigh. In the absence of 
free carbon this loss in weight represents the organic matter which 
was left behind in the total fillers, uncorrected. (See par. 51.) 
In the presence of free carbon the organic matter is arrived at by 
subtracting the amount of free carbon from this ignition loss. 
Subtract the organic matter from the total fillers, uncorrected 
(see par. 51), and report the results as total fillers, corrected. 

56597 °—21-8 



114 Circular of the Bureau of Standards 

53. Sulphur in Total Fillers.—The total fillers, corrected, 
include any sulphur combined with the mineral fillers. It is 
thus necessary to make a correction. This is obtained by 
dissolving in cymene a separate 0.5-gram sample previously 
extracted with acetone. The solution is diluted with petro¬ 
leum ether, filtered, and the residue washed as described in 
the determination of the total fillers. The flask and crucible 
are dried and the pad removed from the crucible and placed 
in the flask. Any fillers which adhere to the sides of the crucible 
are transferred by means of a swab of moistened asbestos. 
Then place the crucible in the mouth of the flask, wash with two 
5 cc portions of nitric acid saturated with bromine (see par. 15) and 
finally with a fine stream of water. Cover the flask and allow it 
to stand in the cold 30 minutes, then on the steam bath for 30 
minutes, evaporate to dryness, add 3 cc of concentrated hydro¬ 
chloric acid, and repeat the evaporation to get rid of any nitrates. 
If a qualitative test shows that there is no barytes present in the 
residue from the total sulphur determination, add hot water to 
the residue in the flask and heat on the steam bath. Filter off the 
asbestos and determine the sulphate in the acidified filtrate. 
(See par. 25.) If barytes is found, determine the sulphur by the 
fusion method for total sulphur. (See par. 27.) Subtract sulphur 
in total fillers so obtained from total fillers, corrected (see par. 52), 
and report the result as total fillers, sulphur free. 

54. Antimony Stocks.—The following determinations are car¬ 
ried out when rubber containing antimony sulphides is to be 
analyzed: Acetone extract (see par. 22); chloroform extract (see 
par. 23); alcoholic-soda extract (see par. 24); free sulphur (see par. 
25); total sulphur (see par. 26); total fillers, uncorrected (see par. 
51); total fillers, corrected (see par. 52); sulphur in total fillers (see 
par. 53); specific gravity (see par. 21). Subtract from 100 per 
cent the sum of the following: Acetone extract, corrected; chloro¬ 
form extract; alcoholic-soda extract; total sulphur; and total fillers, 
sulphur free. Call the figure thus obtained rubber hydrocarbon 
by difference. 

55. Decomposable Fillers.—When decomposable fillers are 
known to be present, the sample should be analyzed as outlined 
for antimony stocks. (See par. 54.) It may be found advisable 
in some cases to use the Joint Rubber Insulation Committee’s 
procedure when analyzing stocks of this type. 


The Testing of Rubber Goods 115 

56. Cork, Leather, Wood Pulp, and Vegetable Fibers: 10 Samples 
of rubber soles, special packings, and other products for special 
purposes which contain the ingredients given above must be 
analyzed according to the procedure for total fillers. (See Par. 
51.) The percentage of rubber hydrocarbon is obtained as the 
difference between 100 per cent and the sum of the following: 
Acetone extract, corrected; chloroform extract; alcoholic-soda 
extract; total sulphur; and total fillers, sulphur free, not corrected. 
The figure for rubber hydrocarbon will not be absolutely accurate 
because of the impossibility of correcting for the organic matter 
which is left behind in the total fillers, uncorrected. The error 
from this source is dependent upon the nature of the stock, but 
will usually be in the neighborhood of 1 per cent. 

57. Cellulose. 10 —Treat a 0.5-gram sample of rubber with 25 cc 
of freshly distilled cresol (b. p., 198° C) on the electric hot plate 
for four hours at 165° C. Allow to cool and add 200 cc of petro¬ 
leum ether very slowly and with constant agitation. After the 
solution has settled completely, filter through a Gooch crucible and 
wash three times with petroleum ether. Wash very thoroughly 
with boiling benzene and finally with acetone. Treat the con¬ 
tents of the flask with hot 10 per cent hydrochloric acid and 
transfer the entire contents to the Gooch crucible with the aid of 
a “policeman.” Continue to treat with hot 10 per cent hydro¬ 
chloric acid until the pad has been washed at least 10 times. 
Wash the pad free from chlorides with boiling water, and run 
small portions of acetone through it until the filtrate is colorless. 
Treat with a mixture of equal parts of acetone and carbon bisul¬ 
phide in the same manner. Wash with alcohol and dry for 1 
hour and 30 minutes at 105° C. Remove the pad from the cru¬ 
cible with the help of a pair of sharp-pointed tweezers, using the 
underneath portion of the pad as a swab to clean the sides of the 
crucible, and place all of this material in a tared weighing bottle. 
Replace in the drying oven for about 10 minutes, cool, and weigh. 
Weight of weighing bottle, pad, insoluble fillers, and cellulose- 
weight of weighing bottle = weight of pad, insoluble fillers, and 
cellulose. Transfer the contents of the weighing bottle to a 50-cc 
beaker and pour over it 15 cc of acetic anhydride and one-half cc 
of concentrated sulphuric acid and allow to digest for at least 
one hour on the steam bath. After the mixture has cooled thor¬ 
oughly, dilute with 25 cc of 90 per cent acetic acid and filter 


10 Epstein and Moore, B. S. Tech. Paper, No. 154- 






116 Circular of the Bureau of Standards 

through a weighed Gooch crucible. To guard against traces of 
the material being carried through, this filtration as well as the 
ones to follow must be very slow and only gentle suction can be 
used. Wash with hot 90 per cent acetic acid until the filtrate 
comes through absolutely colorless and then wash about four 
times more. Wash with acetone about five times. After having 
taken care that all of the material has been washed out of the 
beaker in which the acetylation took place, remove the crucible 
from the funnel, clean the outside thoroughly, and dry for two 
hours at 150° C; cool and weigh. Original weight of crucible+ 
weight of pad, fillers, and cellulose — weight of crucible after acety¬ 
lation = cellulose. 

58. Total Antimony. 11 —When a qualitative analysis indicates 
that antimony is present, extract a 0.5-gram sample with acetone 
for eight hours to remove free sulphur, rubber resins, mineral oils 
or waxes, and part of any bituminous substances or vulcanized 
oils. If the extract indicates that mineral oils or substitutes have 
been used, the sample must be further extracted with chloroform 
until the chloroform in the extraction bucket is colorless. Com¬ 
pletely evaporate off the solvent from the rubber sample in a 
vacuum desiccator. Transfer the sample to a tared 150-cc lipped 
assay flask, add 25 cc of cymene, and heat on an electric hot 
plate or in an electric oven at 130 to 140° C until the rubber has 
completely dissolved, as shown by the fillers settling out and the 
supernatant liquor becoming almost clear. Remove the flask, 
allow the solution to cool, dilute with 100 cc of petroleum ether, 
and allow the solution to stand overnight. Prepare a Gooch cru¬ 
cible with a tight asbestos pad, using asbestos which has been 
previously treated with alkali, concentrated hydrochloric acid, 
and finally water. Carefully wipe off the crucible, dry, ignite, 
and weigh. 

For the best success it is necessary to filter slowly by means 
of gentle suction. The solution is now filtered, keeping as much 
as possible of the fillers in the flask. Wash by decantation with 
petroleum ether until the filtrate is colorless. At least five wash¬ 
ings should be made. Follow with at least five washings with 
boiling benzene, then with acetone, and finally with alcohol until 
the filtrate is colorless. Carefully wipe off the sides of the flask 
and crucible, dry for one hour at ioo° C, cool, and weigh. Sub¬ 
tract the weight of the empty crucible and flask and record the 
difference as total fillers, uncorrected. 


11 Collier, Levin, and Scherrer, Rubber Age and Tire News, 8, 3, pp. 104-105; 1920. 



The Testing of Rubber Goods 117 

Add 30 cc of concentrated hydrochloric acid to the tared assay 
flask and shake until all the antimony sulphide has gone into 
solution. Filter very slowly 5 cc of this solution at a time through 
the tared Gooch crucible by means of very gentle suction, and 
collect the filtrate in a 400 cc beaker. Then wash with boiling 
water and by use of a “ policeman ” transfer the entire contents 
of the flask to the crucible; set aside the Gooch crucible for the 
determination of the total fillers. (For the determination of 
total fillers, corrected, follow paragraph 52.) Dilute the filtrate 
in the beaker to about 250 cc with hot distilled water and pass in 
hydrogen sulphide until the antimony has been completely precip¬ 
itated. The antimony may be determined by either of the fol¬ 
lowing methods: 

(1) Filter off the antimony sulphide and test the filtrate to see 
if the precipitation was complete. Wash the precipitate with 
hydrogen-sulphide water and transfer the precipitate to the 
filter paper. It is frequently difficult to entirely transfer the anti¬ 
mony sulphide to the filter paper, in which case place 20 cc of con¬ 
centrated hydrochloric acid in the beaker and set aside tem¬ 
porarily. Transfer the antimony sulphide and the filter paper to 
a Kjeldahl flask, add 12 to 15 cc of concentrated sulphuric acid, 
add 5 grams of potassium sulphate, place a funnel in the neck of 
the flask, and heat until the solution becomes colorless. Wash 
the funnel and dilute the solution to about 100 cc with water, 
add 1 to 2 grams of sodium sulphite, transfer the hydrochloric 
acid in the beaker in which antimony sulphide was precipitated 
to the Kjeldahl flask, and boil until the sulphur dioxide is all 
driven out. This may be determined by the use of starch-iodate 
paper. Dilute to 250 to 275 cc with distilled water, cool to 10 to 
15 0 C, and titrate with permanganate solution until a faint pink 
color is obtained. 

(2) With the aid of a Witt plate prepare a suitable asbestos 
pad in the bottom of a carbon funnel, filter, and wash as described 
under (1). Transfer the plate, pad, and precipitate to a 600 cc 
Erlenmeyer flask by means of a glass rod pushed up through the 
stem of the funnel. Remove any precipitate adhering to the 
original beaker or to the funnel by washing with the hydrochloric 
acid. Wash the beaker and funnel with hot distilled water, 
dilute the solution to 250 to 275 cc with distilled water, add 12 cc 
of concentrated sulphuric acid, boil the solution until no test for 
hydrogen sulphide is obtained with lead acetate paper, cool to 


118 Circular of the Bureau of Standards 

io to 15 0 C, and titrate against the standard permanganate as 
above. 

59. Ash Analysis. 12 —The results of ash analysis give only an 
indication of the fillers that were used, since we can determine 
only the metallic elements with any degree of certainty, the acid 
radicals of the compounds having undergone at least partial 
decomposition. 'It is recommended when accurate information 
as to the acid radicals is desired and when antimony and lead are 
to be determined that the total fillers, uncorrected (see par. 51), 
be analyzed. The more or less general procedure for ash analysis 
is as follows: Digest the ash in a casserole with concentrated 
hydrochloric acid for one-half hour. Dilute with water, filter off 
the insoluble residue, and set the filtrate aside. Fuse the residue 
with five parts of sodium carbonate, and add the above filtrate to 
the fused mass. Then, if necessary, add sufficient hydrochloric 
acid to make the solution acid. Evaporate the solution to dry¬ 
ness in an evaporating dish, cool, drench with concentrated hydro¬ 
chloric acid, add 10 cc of water, and digest on the steam bath for 
10 minutes. Filter, wash the precipitate with water, and evapo¬ 
rate the filtrate to dryness on the steam bath, extract with hy¬ 
drochloric acid as above, but with the allowance of only a few 
minutes, and filter the solution once more through a second and 
smaller filter. Slowly dry the two papers and their contents, 
char, and ignite in platinum, finally over the blast for 10 min¬ 
utes, cool, and weigh. Treat the residue with hydrofluoric acid 
and a few drops of concentrated sulphuric acid, carefully heat, 
and finally ignite, cool, and weigh. The loss represents the silica. 
Analyze the filtrate from the silica in the usual manner, precip¬ 
itating first with hydrogen sulphide in slightly acid solution, with 
ammonia in the presence of an excess of ammonium chloride, with 
ammonium sulphide, with ammonium carbonate, and finally with 
disodium hydrogen phosphate. The procedure for analyzing the 
precipitates obtained with these reagents can be found in any 
good book on quantitative analysis. 

(d) Joint Rubber Insulation Committee Method 13 

60. Acetone Extract.—Determine as under paragraph 22. 

61. Unsaponifiable Material.—Determine asunder paragraph37. 

62. Hydrocarbons, A.—Determine as under paragraph 38. 

12 The analysis of silicate and carbonate rocks, by W. F. Hillebrand, Bulletin 700, U. S. Geological 
Survey. 

13 Report of the Joint Rubber Insulation Committee, April, 1917. 



The Testing of Rubber Goods 


119 

63. Hydrocarbons, B.—Evaporate the alcohol from the flask 
containing the alcohol-soluble unsaponifiable material (see par. 61) 
and 25 cc of carbon tetrachloride and transfer to a separatory 
funnel. Shake with concentrated sulphuric acid, drain off the 
discolored acid, and repeat with fresh portions of acid until there 
is no longer any discoloration. After drawing off all the acid, wash 
the carbon-tetrachloride solution with repeated portions of water 
until all traces of acid are removed. Transfer the carbon-tetra¬ 
chloride solution to a weighed flask, evaporate off the solvent, and 
dry the flask to constant weight at 95 to ioo° C. Cool in a 
desiccator and weigh. 

64. Free Sulphur.—Determine as under paragraph 39. 

65. Chloroform Extract.—Determine as under paragraph 23. 

66. Alcoholic Soda Extract.—Determine as under paragraph 24. 

67. Rubber Hydrocarbons.—To the flask containing the rubber 
residue from the alcoholic-potash extraction add 25 cc of con¬ 
centrated hydrochloric acid and sufficient water to make the total 
volume 150 cc. Heat for one hour at about ioo° C, decant the 
supernatant liquid through a hardened filter paper on a Buchner 
funnel about 7 cm in diameter, using suction. Wash the residue 
with 25 cc of hot water and decant. Repeat twice this treatment 
with water and hydrochloric acid. The rubber should then be 
white and free from dark specks of undissolved fillers. Add 150 cc 
of hot water to the flask, heat for 15 to 30 minutes, and decant; 
repeat this procedure until the filtrate is free from chlorides. 
Bring all the rubber on the filter paper and suck as dry as possible. 
Add 25 cc of 95 per cent alcohol to displace any water adhering to 
the rubber and again suck dry. Transfer the rubber to a tared 
weighing bottle, dry for one hour at 95 to ioo° C, cool in a vacuum 
desiccator under reduced pressure, and weigh. Repeat the drying 
for one-half hour periods until constant weight is attained. Call 
this weight B. 

68. Take 0.5000 of B, calling this C, ignite in a porcelain cruci¬ 
ble, and when cool weigh the ash, calling this weight D. 

69. Determine the sulphur in D as follows: Add to the residue 
in the crucible about 10 cc of concentrated nitric acid saturated 
with bromine, cover with a watch glass, and heat on the steam bath 
for 15 to 30 minutes. Remove the cover and evaporate to dryness. 
Add about 2 cc of water and 5 grams of 1 :i potassium nitrate and 
sodium carbonate. Dry in an oven or on a steam bath and then 
fuse until all organic matter has been destroyed. When cool, 
place the crucible in a beaker, cover with about 250 cc of water, 


120 


Circular of the Bureau of Standards 


and heat until the melt is dissolved. Filter off from insoluble 
matter and wash thoroughly. Add 7 to 8 cc of concentrated 
hydrochloric acid, cover, and heat to boiling. Test for acidity 
with congo-red paper. Add slowly a slight excess of hot 10 per 
cent barium-chloride solution. Allow to stand overnight, filter, 
wash, ignite, weigh the barium sulphate, and calculate to sul¬ 
phur. (Call this weight E.) 

70. Take a second portion of 0.5000 gram of B (call this F) and 
determine the sulphur (call this G) by the same method as pre¬ 
scribed for determining the sulphur in D. 

71. The weight originally taken for the acetone-extract deter¬ 
mination (that is, the two 2-gram portions) shall be called A . 

72. The rubber hydrocarbons are calculated, using the following 
formula: 

B/ G D-E A 
100 A\ 1 F C ) 

73. Total sulphur.—Determine as under paragraph 26. 

74. Specific Gravity.—Determine as under paragraph 21. 

75. Interpretation of Results.—The rubber shall be considered 
to be the sum of the rubber hydrocarbons, saponifiable acetone 
extract, unsaponifiable resins, chloroform, and alcoholic-potash 
extracts, expressed as percentages. If the chloroform extract is 
over 3 per cent of the rubber so calculated, subtract the excess 
from the rubber. If the alcoholic potash extract is over 1.8 per 
cent of the rubber as first calculated, subtract this excess also from 
the rubber. 

76. Statement of Results.—The results of the analysis shall be 
stated in the following form: 

Per cent 

Acetone extract. 

Saponifiable acetone extract.,. ... 

Unsaponifiable resins. . 

Waxy hydrocarbons. 

Free sulphur. 

Chloroform extract. 

Alcoholic-potash extract. 

Total sulphur. 

Carbon. 

Rubber. 

Color of acetone extract (60 cc volume). 

Fluorescence in acetone extract solution (present or absent). 

Hydrocarbons A (consistency and color). 

Hydrocarbons B (solid or liquid). 

Color of chloroform extract (60 cc volume). 

Specific gravity. 


















I 2 I 


The Testing of Rubber Goods 

77. (e) ISiotes .—In the event of any determination not falling 
within the limits given in the specifications, a check test should 
be made. It is essential that no material be condemned unless 
the results on which the recommendation for rejection is made 
are established beyond reasonable doubt. 

78. If the percentage of free sulphur in insulation compounds 
(see par. 39) is greater than that permitted by specification, the 
check test may be made according to paragraphs 22 and 25. 
The time for extraction shall be eight hours. 

79. In all sulphur determinations unnecessary excess of acid 
should be avoided. Barium sulphate is appreciably soluble in 
hot acid solutions, and the amount so dissolved should be kept 
as small as possible. Before adding the barium-chloride solution 
always test for acidity with congo paper. Considerable time can 
be saved if the contents of the crucible is thoroughly dried in an 
oven at about 125 0 C before fusing. Care should be taken to 
prevent the creeping of the salts over the edge of the crucible. 
A qualitative test for the presence of barytes can be made on 
the residue left on the filter paper in the determination of the 
total sulphur. 

80. Whenever the procedure for total fillers is used in order to 
determine total fillers, every solvent used must have been pre¬ 
viously filtered, as there is considerable danger of the introduc¬ 
tion of foreign matter from this source. Likewise in the deter¬ 
mination of cellulose, extra precautions must be taken to be 
assured that all reagents are absolutely free from residue. 

81. When the procedure for total fillers is followed, the com¬ 
bined filtrates containing cymene, petroleum ether, and benzene 
can be collected and distilled periodically. The first step of the 
distillation is carried out on steam bath. The petroleum ether 
will be recovered at about 50° C. The benzene is recovered as 
the temperature is raised. Finally the water condenser is re¬ 
placed by an air condenser and the cymene is distilled out at about 
175 to 185° C. 

82. When cellulose is determined, the combined acetic anhydride 
and acetic acid filtrates should be collected and distilled periodi¬ 
cally. The distillate can be used for washing in place of the 90 
per cent acid called for in the directions. Cymene can probably 
be substituted for cresol, but as no work has yet been done to 
justify this change cresol is still used. 

83. When a Gooch crucible is used for filtration, care must be 
taken to prepare the asbestos properly. This should always be 


122 Circular of the Bureau of Standards 

done as follows: The asbestos is cut fine with shears, digested 
with a io per cent caustic-soda solution, washed with water, and 
then digested with concentrated hydrochloric acid for a few 
hours on the steam bath. After it has been washed comparatively 
free from acid by decantation, the asbestos is shaken up with 
water and the resulting mixture is used in preparing the pads. 
The Gooch crucibles should be ignited at about 8oo° C and are 
then ready for use. 

84. If it is found impracticable to run blanks on every bottle 
of reagents used, the danger of impurities seriously affecting the 
results should not be lost sight of. It will probably suffice to 
prepare reagents in large quantities and make tests on each fresh 
lot. When there is any doubt about the matter, a blank should 
always be run. 

VI. TESTING OF FABRICS 

1 . NORMAL ATMOSPHERE AND MOISTURE CONTENT 

All fabric tests are made under “ normal atmosoheric condi¬ 
tions;” that is, a relative humidity of 65 at a temperature of 70° 
F (21 0 C) after the test specimens have been exposed in this 
atmosphere. 

When it is not practicable to maintain a normal atmosphere, 
the tests may be performed under existing humidity conditions 
and the results corrected to the basis of a content of 6 per cent of 
moisture by multiplying by the following factor: 

100 

100 + 7 (percentage of moisture — 6) 

The factor will be less than unity when the per cent moisture is 
greater than 6, and vice versa. 

The moisture content shall be determined from the tensile-test 
specimens. Weigh the specimens and immediately determine the 
tensile strength, then place the broken samples (entire) in a ven¬ 
tilated drying oven at 105 to iio° C (221 to 230° F) until the 
weight is constant. The moisture present at the time of testing 
shall be calculated on the basis of the dry weight. 

2 . WEIGHT 

Three weight samples of not less than 4 square inches each shall 
be cut from the fabric in such a manner that they will be repre¬ 
sentative of the fabric, and exposed in ‘‘normal atmosphere” for 



123 


The Testing of Rubber Goods 

at least three hours and then weighed in this atmosphere. The 
average of the three weights shall be considered to be the weight 
of the fabric. 

3. THREADS PER INCH 

The number of threads per inch shall be determined by counting 
a space of not less than i inch in at least five different places in 
both the warp and the filling directions. The average of the five 
determinations shall be considered to be the threads per inch or 
thread count. 

4. TENSILE STRENGTH 

The tensile strength shall be determined from strips selected as 
follows: Starting in the center of the test sample cut three strips 
6 inches long by i % inches wide parallel to and in the direction of 
the warp and three strips parallel to and in the direction of the 
filling. These specimens shall be raveled until exactly i inch of 
woven fabric remains by pulling out approximately the same 
number of threads from each side. 

An inclination balance or pendulum type of testing machine of 
an approved type and capacity shall be used in determining the 
tensile strength. The distance between the jaws or clamps of the 
testing machine at the beginning of the test shall be 3 inches and 
the jaws or clamps shall separate at a uniform rate of 12 inches per 
minute during the test. 

For detailed information concerning the testing of textiles see 
this Bureau’s Circular 41 and Technologic Paper No. 68. 

VII. APPENDIX 
1. LIST OF SPECIFICATIONS 

An important part of this Bureau’s work on rubber consists in 
the development of specifications for the various Government 
departments. The following list contains those specifications that 
have been completed: 

Recommended specifications for pneumatic tires, solid tires, and inner 
tubes. B. S. Circular No. 115. 

Fire hose. B. S. Circular No. 114. 

Rubber tubing. 

Rubber gloves. 

Hospital sheeting—written for the Field Medical Supply Depot, U. S. 

Army. 

Rubber ring cushions. 

Hot-water bottles. 

Rubber stoppers. 

Rubber jar rings—written for the States Relation Service, Department 
of Agriculture. 


124 


Circular of the Bureau of Standards 

This Bureau cooperated with the War Department Committee 
for the Standardization of Mechanical Rubber Goods in the prepa¬ 
ration of the following: 

War Department 


number 

General specifications for mechanical rubber goods. 333 —i—i 

Air brake and signal hose and gaskets. 333-1-3 

Gas hose. 333-1-2 

Dredging sleeves. 333-1-4 

Chemical engine hose.r 333-1-5 

Diver’s hose. . 333-1-6 

Cotton rubber-lined fire hose. 333-1-7 

Gasoline hose. 333— J —8 

Radiator hose. 333-1-9 

Pneumatic hose. 333-1-10 

Steam hose. 333-1-n 

Suction hose. 333-1-12 

Corrugated tender hose. 333-1-13 

Water hose. 333-1-14 

Laboratory tubing. 333-1-15 

Cloth-insertion tubing. 333-1-16 

White machine rubber tubing. 333-1-17 

Cloth-insertion rubber packing. 333-2-1 

Diaphragm packing. 333-2-2 

Red sheet packing. 333 _2_ 3 

Tuck’s packing. 333-2-4 

Rubber transmission belting. 333 ~ 3 _I 

Balata. 333 ~ 3 _2 

Rubber valves. 333-1-2 

Bumpers. 333 -J -3 

Rubber tips for flexible metallic hose. 333-1-1 

2 . BIBLIOGRAPHY 


This circular is concerned chiefly with the problems connected 
with the testing of rubber goods. There will be many who are 
interested in other phases of the rubber industry and for their 
benefit a partial list of publications is given below. 

PUBLICATIONS OF THE BUREAU OF STANDARDS 

Scientific Paper No. 174. The determination of total sulphur in india rubber, C. E. 
Waters and J. B. Tuttle. Also in J. Ind. Eng. Chem., 3 , 734, 1911. 

Technologic Paper No. 35. Combustion method for the direct determination of 
rubber, L. G. Wesson. 

Technologic Paper No. 45. A study of some recent methods for the determination of 
total sulphur in rubber, J. B. Tuttle and A. Isaacs. Also in J. Wash. Acad., 5 , 
2 35 - 2 3 6 - I 9 I 5'» J- Ind. Eng. Chem., 7 , 658-663, 1915. 

Technologic Paper No. 64. Determination of barium carbonate and barium sulphate 
in vulcanized rubber goods, J. B. Tuttle. Also in J. Ind. Eng. Chem., 8, 324-326, 

1916. 

Technologic Paper No. 136. Determination of free carbon in rubber goods, A. H. 
Smith and S. W. Epstein. Also in J. Ind. Eng. Chem., 11 , 33-36, 1919. 

Technologic Paper No. 145. Direct determination of india rubber by the nitrosite 
method, J. B. Tuttle and Louis Yurow. Also in India Rubber World, 57 , 17-18, 

1917. 




























The Testing of Rubber Goods 125 

Technologic Paper No. 162. Extraction of rubber goods, S. W. Epstein and B. E- 
Gonyo. Rubber Age and Tire News, 6, 445-447, 1920. 

Technologic Paper No. 154. Determination of cellulose in rubber goods, S. W. 
Epstein and R. L. Moore. Also in Rubber Age ^nd Tire News, 6, 289-293, 1920. 

Technologic Paper No. 68. Standardization of automobile fabric testing, Walter 
S. Lewis and Charles J. Cleary. 

Circular No. 41, 3d Edition. Testing of textile materials. 

The sampling of rubber goods, J. B. Tuttle, J. Ind. Eng. Chem., 5 , 618, 1913. 

An improved extraction apparatus, T. B. Ford, J. Am. Chem. Soc., 34 , 552-553, 
1912. 

Detection of glue in rubber goods, S. W. Epstein and W. E. Lange, India Rubber 
World, 61 , 216—217, 1920. 

Determination of antimony in rubber goods, S. Collier, M. Levin, and J. A. 
Scherrer, Rubber Age and Tire News, 8, 104-105, 1920; India Rubber Journal, 64 , 
580, 1921; Rubber Engineering, Production, 2 , 6-13, 1921. 

An improved method for the determination of total sulphur in rubber goods, 
Rubber Age and Tire News, 9 , 47-48, 1921. 

BOOKS 

The chemistry of India rubber, by Carl Otto Weber; Charles Griffin & Co. (Ltd.), 
London; 1902. 

Rubber, by Philip Schidrowitz; Methuen & Co. (Ltd.), London; 1911. 

Rubber tires, by Henry C. Pearson; The India Rubber Publishing Co., New York; 
1906. 

Crude rubber and compounding ingredients, by Henry C. Pearson; The India 
Rubber Publishing Co., New York; 1918. 

India rubber laboratory practice, by W. A. Caspari; MacMillan & Co. (Ltd.), 
London; 1914. 

Der kautschuk und seine prtifung, by F. W. Hinrichsen and K. Memmler; S. 
Hirzel, Leipzig; 1910. 

Die analyse des kautschuks, etc., by Rudolf Ditmar; A. Hartleben, Leipzig; 1909. 
The manufacture of rubber goods, by Adolf Heil and W. Esch, translation by Edward 
W. Lewis; Charles Griffin & Co. (Ltd.), London; 1909. 

The chemistry of rubber, by B. D. Porritt; D. Van Nostrand Co., New York; 1914. 
The chemistry of the rubber industry, by Harold E. Potts; Constable & Co. (Ltd.), 
London; 1912. 

Rubber, its production, chemistry, and synthesis, by A. Dubose and A. Luttringer; 
Charles Griffin & Co. (Ltd.), London; 1918. 

India rubber and gutta-percha, by T. Seeligmann, G. L. Torrilhon, and H. Falcon- 
net; Scott, Greenwood & Son, London; 1910. 

Plantation rubber and testing of rubber, by G. Stafford Whitley; Longmans, 
Green & Co., London and New York, 1921. 

Rubber manufacture, by H. E. Simmons; D. Van Nostrand Co., New York, 1921. 

PERIODICALS 

Devoted Solely to the Interests of the Rubber Industry 

India Rubber World. 

The Rubber Age and Tire News. 

India Rubber Journal. 

Gummi-Zeitung. 

Le Caoutchouc et la Gutta Percha. 

Having Abstracts or Occasional Articles on Rubber 

Journal of Industrial and Engineering Chemistry. 

Chemical Abstracts. 

Journal of the Society of Chemical Industry. 

The Analyst. 

Kolloid-Zeitschrift. 


126 


Circular of the Bureau of Standards 


3 . TABLE OF SPECIFIC GRAVITIES 


Materials 

Mini¬ 
mum 14 

Maxi¬ 

mum 14 

Aver¬ 
age 15 

Materials 

Mini¬ 
mum 14 

Maxi¬ 
mum 14 

Aver¬ 
age 15 

Asbestine. 

2. 70 

2. 82 

2. 85 

Magnesium carbonate.... 

3. 00 

3. 07 


Acetone. 


. 797 

. 797 

Mica. 

2. 80 

3. 2 


Aluminum silicate. 

2. 61 

3. 02 


Ozocerite . 

. 90 

. 95 


Aluminum flake. 

2. 56 

2. 65 


Paraffin. 

. 869 

. 91 


Aniline . 

1. 00 

1. 03 


Paraffin oil. 

. 90 



Antimony, red. 

2. 87 



Paraffin wax. 

. 91 



Antimony, golden. 

2. 57 

2. 90 


Palm oil. 

. 94 



Antimony, crimson. 

3. 11 

3. 70 


Petroleum. 

. 89 

. 90 


Antimony, black. 

4. 80 



Pine tar. 

1. 05 



Asphalt. 

. 99 



Pitch, roofer’s. 

1. 23 

1. 25 


Asphalt, liquid. 

. 99 



Pitch, coal tar. 

1. 28 



Asphalt, Trinidad. 

1. 2 



Prussian blue. 

1. 96 



Arsenic, yellow. 

2. 75 



Rosin. 

1. 05 

1. 08 


Balata. 

1. 05 



Red oxide. 

4. 82 

5. 16 


Barytes. 

4. 38 

4. 92 

4. 45 

Resilio. 

2. 60 



Benzene (benzol). 



. 745 

Rubbers: 




Bitumen. 

1. 07 

1. 16 


Accra flake. 

1. 02 



Black substitute. 

1. 10 



All varieties. 

. 95 



Brown substitute. 

1. 07 

1. 32 


Crepe No. 1, first latex.. 

. 913 

. 92 


Bone black. 

2. 20 

2. 32 


Amber crepe. 

. 92 

. 928 


Carbon black. 

1. 68 

1. 89 

1. 81 

Roll brown crepe. 

. 95 



Camphene. 

. 865 

. 875 


Caucho ball. 

. 915 

. 922 


Castor oil. 

. 958 



Cameta. 

.916 

. 92 


Candelilla wax. 

. 99 



Caoutchouc. 

.92 ' 

. 96 


Carnauba wax. 

. 995 



Assam. 

.967 



Ceresin. 

. 918 

. 922 



. 916 



Chrome green. 

5. 24 

5.44 



. 928 



Chloroform. 

1. 52 


>3.90-5.08 


. 929 



Carbon tetrachloride. 

1. 61 


1. 60 

Guayule. 

. 975 

. 976 


China clay. 

2. 2 

2. 6 

2. 62 

Guayule, extracted. 

. 995 



Cotton. 

1. 47 

1. 55 


Fine Para. 

. 915 

. 95 


Cork. 

. 24 

1. 00 


Madagascar. 

. 915 



Cottonseed oil. 

. 922 

. 93 


Mozambique. 

. 939 



Corn oil. 

. 926 

. 93 



. 918 



Coal tar. 

1. 05 

1. 27 


Coarse Para-. 

. 95 



Carbon bisulphide. 

1. 26 

1. 29 


Vulcanized Hevea, hard 

. 95 



Chrome yellow, light. 

6. 41 



Vulcanized Hevea, soft. 

. 92 



Chrome yellow, medium. 

5. 73 

5. 84 


Smoked sheets. 

. 909 

. 95 


Chrome yellow, deep. 

5. 91 

6. 08 

6. 00 

Sernamby. 

. 918 



Fuller’s earth. 

1. 80 

2. 70 


Sierra Leone. 

. 923 



Fossil flour. 

2. 60 



Sapori. 

. 928 



Gasoline, 72 to 75° Be.... 

. 70 

. 707 


Senegal. 

. 929 



Glycerin. 

1. 25 

1.30 


Singapore. 

. 937 



Glue. 

1. 30 



West Indies Islands.... 

. 935 



Glass, powdered. 

2. 49 



Black caoutchouc. 

. 945 



Graphite. 

1. 95 

2. 32 

2. 36 

Soapstone. 

2. 23 

2. 70 


Gutta-percha. 

. 96 

1. 00 


Sulphur chloride. 

1. 69 

1. 71 


Indian red. 

4. 80 

5. 25 


Sulphur 

1. 96 

2. 07 


Infusorial earth. 

1. 66 

1. 95 



1 95 

2. 25 


Kaolin. 

2. 75 




1. 30 



Lampblack. 

1. 53 

1. 75 



2. 00 

2 78 

2. 84 

Lead, blue. 

6. 40 




2. 30 

2. 40 

2. 35 

Lead, red. 

8. 17 

9. 00 

8. 80 


1 Qfi 

2 07 

3.05 

Lead chromate. 

5. 65 

6. 12 


Vermilion. 

7. 89 

8. 10 

Lead sulphate. 

6. 08 

7. 7 


Vaseline. 

. 84 

. 945 


Lead, white. 

6. 10 

6. 75 

6. 81 


1 43 

1. 46 


Lead, sublimed. 

6. 20 

6. 30 

6. 41 

White substitute... 

1. 04 

1. 14 


Litharge. 

8. 9 

9. 52 

9. 40 


2 60 

2 72 

2.71 

Linseed oil. 

. 94 


j.932-.942 


1. 00 

1. 08 

Lime. 

2. 21 

2. 28 

Xvlidine. 

. 92 

. 994 


Lithopone. 

3. 60 

4. 25 

4. 30 

Yellow ocher. . 

6. 00 



Magnesia. 

2. 16 

3. 65 



5 38 

5. 60 

5. 66 

Mineral rubbers. 

1. 00 

1.06 ■ 


Zinc sulphide. 

3. 5 

Montan wax. 

1.04 



Zinc sulphate. 

3. 62 



Magnesium carbonate, 




Zinc carbonate. 

4. 42 

4. 45 


light. 

1. 74 



Zinc, leaded. 

5. 64 


5. 95 








14 These results were obtained from a table compiled by H. P. Gumey, of the Boston Belting Co., who 
obtained them from the work of H. P. Gumey, G. H. Ellinwood, S. Collier, and H. J. Persoon, while 
working at different factories in the rubber industry; from I.andolt-Bemstein, Physikalisch-chemische 
cabellen; Van Nostrand’S'Chemical Annual; The chemistry of india rubber, by C. O. Weber; Oil analy¬ 
sis, by Gill; Paints, colors, oils, and varnishes, by Hurst; A. H. King, The Chemical and Metallurgical 
Engineering Journal, June, 1917 , and C. S. Redfield, The Rubber Age, June n, 1917 . 

15 Taken from “Bulking values and yields of pigments and liquids used in paint and enamel manufac- 
ure,” by Henry A. Gardner and Harold C. Parkes. 








































































































































































































































































The Testing of Rubber Goods 127 

4. DETERMINATION OF THE SPECIFIC GRAVITY, COST PER POUND, AND 
COST PER CUBIC FOOT OF A RUBBER COMPOUND 


Ingredients 

Weight 

Specific 

gravity 

Relative 

volumes 

Cost 

Rubber. 

Pounds 

50.0 

0.94 

53.2 

Dollars 

15.00 

Zinc oxide. . 

35.0 

5.60 

6.25 

3.51 

Mineral rubber. . 

4.0 

1.05 

3.81 

.12 

Litharge. 

8.0 

9.40 

.85 

1.20 

Sulphur. 

3.0 

2.06 

1.46 

.03 


Totals. . 

100.0 


65.57 

19.86 




„ . \\ eight 100.0 T . » . . , , , . . 

Density=vrv j5 —== =1.1:2=;. It is the universal practice, m the rubber mdus- 

J Volume 65.57 J J 

try, to call this “specific gravity.” 

„ A , Cost $19.86 

st per poun Weight (pounds) 100.0 , ** oi 9^6. 

Cost per cubic foot=cost per poundXspecific gravity of the compoundXweight per 
cubic foot of water=o.i986Xi-525X62.3o=$i8.87. 






























































































jg£$Suj 




'Jr; 







•j. 





£ 4 ' ' V 


- 











